Is anyone else having problems with a changing IP address?

I have had TMobile for about 6 mo. Was fine at first. Then when I tried to use our schools gradebook it would constantly take me back to the login screen. I have seen others post about not undefined, undefined… I had that as well. 

After reading all the "potential" problem solving solutions. I have tried almost all except for getting the business acct. Which is what I was also told when I called TM. 

Long story short… whomever suggested tethering your phone with a hot spot and a VPN… it worked. Well, after a couple of tries because of having the exact setting on the phone. But I can now log in and save grades from home. Not convenient, but it is working.