@Ninjah9 - as it's working fine on 4G, it looks like you might have an issue with the WiFi, even though you think it's working fine with the other devices.
Do this on a laptop connected to the same WiFi as your phone:
- press "WinKey+R"
- type "cmd" and press Enter
- in the command prompt window type "ping www.google.com -t" and press Enter
- leave it to run for about half an hour
- when you want to finish the test, press "Ctrl+C" to stop it
You should now see something like this at the end "Packets: Sent = 30, Received = 30, Lost = 0 (0% loss)"
If you lose a lot of packets it means that your internet connection is not stable.
PS. If you're using a Mac, skip the first 2 steps and open the Terminal app.
It also depends upon the phone... my iPhone6 takes between 1-2 minutes for the game to fully load while on my XR it takes about 10 seconds.