I hate to ask you to list the everything you tried to see if there was something you didn’t that I could suggest that you missed.
The most painful is to uninstall and reinstall the game.
If you have an android phone, clear the cache.
Reboot your phone or ipad. I’ve have instances every so often, I can’t get in… I can’t get in and I have good signal and all. I reboot my phone and am able to connect. This happens away from home out on my bike.
Connect to wi-fi and try to load the game. This issue I have often and works to at least know I can get in. Low signal can cause issues so wi-fi ensures good data. Also make sure your in close range of the wi-fi. Even in my house there are spots where it doesn’t get through the walls even though I’m close to the router.
Another issue I have is I have a Sprint tower booster I got from Sprint for free that my phone will connect to when I’m close or in the house. I occasionally have trouble with data connection through that thing for some reason so I turn it off. Then my phone connects to the phone tower 6 miles away. That has worked for me.