Need Help with Connecting Access Point to TP Link 3420 running as primary router

I have a WISP Connection which has allocated me IP "172.150.0.123"
his Default gateway is "172.150.0.1"
When I set my Network Interface (WIFI/LAN) I need to access his login page at 1.254.254.254 and login manually.
Uptil now I was able to access internet by using the Inbuilt WI-FI Card on laptop but now he has lots of bird nest in his
tower, which are visible and signal strenght has gone down but he will NOT FIX it, so I grabbed a Access Point from him
(Currently under testing) as its no where near TP-Link 5210 and other model.
So as I Already Have a TP-link MR3420, Running DD-WRT, I was able to set it up simply by inserting The LAN Cable coming from
this access point to WAN port in MR3420, however this had issues as it connecting only my laptop (cause IP is x.x.x.123) thru this
connection.
This access point doesn't has its own IP, whatever IP I assign to network adapter is getting detected at WISP login page, if its my allocated IP it connects, if I change the network adapter IP, I get IP mis-match error and unable to login.

If i try to access Internet thru TP-Link Router SSID, I am getting a new IP allocated and Login page comes up and rejects me
cause its not matching IP "172.150.0.123"
I was following a couple of setups and now I am not able to connect this WISP Access Point to this router, Its not even
giving me the login page which used to work till now.
Does anyone here has tips on how to make use of my existing router and able to connect all devices.
Eg: I want router to be appear as IP "172.150.0.123" and once internet is connected all other devices are able to access internet.
It may be a easy step, but I am not a network guy so have limited knowledge about it,
please let me know if there's a easy way to setup this thing.

My friend, you had not mentioned the static ip limitation in the other thread. You need to set up WAN as static ip (use the given ip and DNS) in the router. If your provider employs mac lock in addition to ip restriction, change mac of router to the wifi mac of laptop (there's no harm in doing this even if there's no mac lock). The first device that connects will see the login page and will need to login, all the rest will have internet connectivity without logging in. Make sure to enable DHCP on the router and set laptop to get ip and DNS automatically.

There's no mac binding, as I can set any PC with that IP and login.
Earlier I tried what you have suggested by setting WAN IP to my allocated IP and connecting and it didn't worked out.
Setting PC to auto IP gave me IP Mis-match error !!
Going to Reset the Router to defaults and try again by evening.
By any chance Do I need to separate LAN and WLAN ???

Setting PC to auto ip won't cause a problem as PC is behind router, its ip is never seen by your ISP unless your settings are all wrong or cable has not been put into WAN port of the router. I think you've still got that AP setting which you showed me yesterday. In that setting ip of every device will be visible upstream. As I said yesterday, don't use that.
It's a simple static WAN ip setting in normal router mode that is required, nothing else.

he he he,
I've done 3-4 resets cause I am a mess when it comes to such settings.
Steps I've done
Changed default user/pass
Set my SSID, secured the wi-fi
WAN Setup---> Connection Type "Static IP" with details matching the required IP from WISP.
First hurdle, What would be in "Network setting" just below the "WAN Setup"
By Default its having
Local IP Add: 192.168.1.1
Subnet: 255.255.255.0
Gateway: 0.0.0.0
Local DNS: 0.0.0.0
and
Wireless mode has options AP, Client, Client (Bridge), Adhoc, WDS Station, WDS AP
which one to use ???

The network setting is for the internal network, you can give 192.168.1.1 as gateway and dns. Leave wireless mode in default AP setting.

as soon as I define IP, and apply settings,
I can no longer get to Admin Interface and remain locked out and internet even doesn't works even if i insert the cable from Access Point to WAN Port on this one.

Have you put your laptop to get auto ip? Is the router's DHCP on? Put static ip 192.168.1.50, gateway 192.168.1.1 on the laptop and check.
Also, has the provider given you any other DNS, or do you use Google DNS even on the laptop when connected to the ISP?

Also very important: your network configuration setting must be unbridged, you may have changed it to bridged when you set it up as a true AP.

Thanks again, you have a lot of patience.
trying both option and updating this post in a few minutes...


NOPE, not working, RESET AGAIN.
Edit: 12:25AM
THANKS A LOT MAN, you kept helping me all day long, Thanks Again.

BINGO, you were Right about bridge
I broke a wire and had to re-crimp the LAN Cable,
Connected Laptop thru it,
Internal network is on 192.168 type IP while external is 172.xxx.x..x what isp wants.
Connected a Android and its working too.
Some issue with sites unable to finish loading, but that should be easy to fix.
Doing speedtest.net since 12:25..............................
its 12:31 yet to get completed

You're welcome. With stock firmware this would have been a very simple set up but because DDWRT has such a lot of options, it's easy to go awry somewhere.
You may need to fiddle around with the settings to resolve the issues, most probably some packets being dropped due to some settings. Check if there's any packet loss at pingtest.net Also see if the problem exists on all connected devices or just one. You may also need to look at the mtu settings on the router.
Also check by giving static ip on the laptop in the 192.168.1.x series with gateway 192.168.1.1 and with DNS 8.8.8.8 & 208.67.220.220 also defined on the laptop. If the problem persists you could also try going back to the stock firmware.

Thanks again, Stock would have been easy, but DD-WRT is superb when comes to customizing.
yes Earlier I tried all options, by setting laptop on static IP, or auto, and all devices were having speed issues.
But after adjusting MTU to lower value the problem seems to be solved.
But still I see sometimes pings are lagging a lot, may be ISP issue cause same issue even when connecting laptop directly,
connection gave this result last night
https://www.speedtest.net/my-result/3499074742
^ Decent speed for a 512kbps unlimited connection
and now its
https://www.speedtest.net/my-result/3499774043