"Cannot verify server identity" with Powerschool app
I get a "cannot verify server identity" notification 30-40 times a day and have been getting it since Nov. This is NOT for a mail app, it's for the Powerschool app. I called Apple in Dec, they told me to call my carrier. I called AT&T, they said to contact the developers. After a couple of months of research, the developers told me it's a problem with my phone and they cannot recreate it. I called Apple back today and they said to call the developers.
The server listed in the notification is an old server and is no longer used. I have deleted the app numerous times, I have cleared network settings and reset the phone twice. I still get the error no matter what. It's driving me insane. I'm afraid to get a new phone (even though I don't need one), because I am not sure this won't happen on a new phone.
Help!
iPhone X