On a windows computer open up a command prompt window (type "cmd" into the search bar) and run a ping command for the website. You're not interested in a reply to your ping as some websites won't reply to pings, what you are looking for is if your ISP's domain name server actually recognizes this name and coverts it to the appropriate IP address.
In the following example I used the command "ping google.com" and received a reply of "Pinging google.com [142.251.40.238] with 32 bytes of data:", anything after that doesn't matter as what you want is that number "142.251.40.238". If you don't get a number back but rather something like "Ping request could not find host google.com. Please check the name and try again.", then it's a problem with ISP's DNS server.
Microsoft Windows [Version 10.0.22631.4317]
(c) Microsoft Corporation. All rights reserved.
C:\Users\XXXX>ping google.com
Pinging google.com [142.251.40.238] with 32 bytes of data:
Reply from 142.251.40.238: bytes=32 time=24ms TTL=116
Reply from 142.251.40.238: bytes=32 time=24ms TTL=116
Reply from 142.251.40.238: bytes=32 time=24ms TTL=116
Reply from 142.251.40.238: bytes=32 time=24ms TTL=116
Ping statistics for 142.251.40.238:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 24ms, Maximum = 24ms, Average = 24ms
C:\Users\xxxx>
If you don't get the IP number from the ping, connect your computer to your phone's hotspot and run the command again. When you get the responce with the IP number, write it down, reconnect to your ISP, and then use that number in your browsers address bar instead of the name and it should work.
You could also make changes to your routers settings to specify a different DNS other than the default provided by your ISP. Google's DNS server is 8.8.8.8 and you can follow the instructions here https://cleanbrowsing.org/help/docs/change-dns-on-a-google-wifi-mesh-router/ to change the default DNS to Googles.
