Nokia g-140W airtel fiber ADSL to router

I upgraded my connection and therefore they changed my ADSL to the new router which Nokia G-2425G-A.
I want to use my old ADSL as a router to increase the range for my new router, I looked on the internet disabled DHCP server and then the next step is to change the LAN IP address but I'm unable to do sue to which my main router gives me 50Mbps and the extended router also gives 50Mbps.
Since the LAN IP is not changing it's dividing my internet speed but I don't want it to divide the internet speed instead the extended ADSL>Router should also be giving 100Mbps.

New router admin login - 198.168.1.1
Old router - 198.168.1.1

Now I tried many stuffs from the internet also, I saw yt video he put the LAN IP address 192.168.0.1, even I tried the same but it's giving me an error "lan address and dhcp should be in the same subnet".
Then I read somewhere that you also need to change the Subnet Mask but this time i did not get any error but also the router just stopped working.

Also when I use it that way where it halves my bandwidth it also interferes with my pc LAN connection my network gets very slow and responsive, it also keeps disconnecting and reconnecting to the internet from the router itself.