Odd "cannot find server" errors during most of the day

The problem I am facing is unusual and requires some network expertise. A friend familiar with networking is bewildered how some websites I try to open can be readily openable through tethered Jio Mobile from Windows 10, but in two Linux systems on the same computer using various similar browsers, they are not openable. Some websites, such as Google Search, YouTube, Facebook and others are always openable on my two Linux OS's as they are in Windows. But during the daytime the vast majority of sites reliably open in Windows only. Strengthening the mobile signal by moving to a location with all 4 bars doesn't help.

Maybe this is a DNS problem. A typical immediate chromium browser error message is:

This site can’t be reached
Blender Experimental Builds - blender.org is unreachable.
ERR_ADDRESS_UNREACHABLE

This is the common error during most of the day in Linux, but it never appears in Windows. Oddly, at around 3 PM every day the error does not happen in Linux and I am able to open sites such as Amazon.in for an hour or so. That site throws the above error during most of the day in Linux, though Google, etc. are openable at all times in Linux and Windows.

The problem started about 2 months ago when the covid Internet overload messed up reliable Internet access. Lately a friend with Mac OS is having a similar problem tethering on his Mac laptop and is restricted to using his phone's browser during non-early-morning times. Any insights or suggestions would be appreciated. JioCare suggested clearing the browser DNS settings, which didn't help.

I am really sorry about what you are experiencing.
It appears that the issue only persist with the IPV4 websites.Since all Google website are relying on IPV6 whenever possible and Jio already using IPV6 address for the mobile users,you are able to access all other IPV6 websites.
TBH,I have never read the official documentation of IP addresses,so I cannot help you.
But I can gladly refer the capable person to help... Please help us @varkey.
BTW,did you see if the IPV4 is disabled in Linux setting for the adapter.I was facing these issues when I mistakenly forget to set my IPV4 address to automatic after using static IP routes across the network to access OLT of my LCO.😁

Yes, I was wondering if IPv's might be the problem, but in Ubuntu both IPv4 and IPv6 are set to automatic on my system. Any other ideas?

From a little reading on the Internet it appears IPv4 is not very common these days, at least not enough to cause 90% of sites to not be found. I wonder whether the OS has it's own DNS cache that needs to be flushed, but so far have not found that specific info.

Use following command to flush DNS cache using systemmd-:
$ sudo systemd-resolve --flush-caches
Now,check statistics for cache size-:
$ sudo systemd-resolve --statistics
The cache size must be zero.

Using the systemd-resolve --flush-caches command did reset the cache size to zero, but the problem remains:

www.Amazon.in’s server IP address could not be found.
Try:
Checking the connection
Checking the proxy, firewall and DNS configuration
ERR_NAME_NOT_RESOLVED

Removing Windscribe VPN didn't help.
Clearing the Chromium browser cache and sockets didn't help.
Changing the DNS settings to Cloud Flare's didn't help.
Turning off Settings/Privacy/Connectivity Checking changed the networking icon from a question mark to the normal connection icon, but didn't help.
Restarting after all of the above didn't help.
??

Do you use DNS over TLS ?
Misconfiguration in that case maybe an issue.
Try live USB....😅
Did you restart network manager....🙄
Try it
$ sudo netplan apply
Or,try disabling ipv6-:
$sysctl -w net.ipv6.conf.all.disable_ipv6=1
$sysctl -w net.ipv6.conf.default.disable_ipv6=1
And restart network manager...Or better restart the system.
to re-enable IPV6,just replace 1 by 0 in these 2 commands.Since cache is clear but host cache of chromium is not cleared,you can try it.
Just follow this-:
chrome://net-internals/#dns > "Clear host cache" button.
Did you try any other browser as I had faced the same issue with Chromium on my PC and solved by host cache clearance of browser.
Sorry,I forgot this earlier.It should resolve your issue.You can also disable DNS prefect but I do not exactly remember how to do this....😅

Is there a command to test DoT?
Just downloaded Ubuntu 20.04 iso and put it on a thumb drive but like 18.04 the live version does not have mobile tethering available, only WiFi.
I have tried Vivaldi, another chromium-based browser, and also Firefox but they have the same problem.
Tried the other commands you listed but nothing solved the problem.
Have cleared the browser cache several times and turned on and off prefetch (now called preload) but no improvement.
At this point the strong suspicion is that Jio either has a policy that doesn't favor major Linux distros or its networking is misconfigured in this regard.