Messages take 5 minutes to receive

Ok so I am at my wits end trying to figure this out. I have a Nokia Lumia 635 on AT&T and my girlfriend has a Nokia Lumia 521 (I think...) on T-Mobile. Every day at around 6:53am we text and it takes 5 minutes to get a message from her on my end! At about 6:58am I get the messages and she thinks she is ignoring me and it's really upsetting me and I'm considering switching to another carrier if this keeps up so any help would be appreciated! If it helps, we live in Western North Carolina and there's a lot of dead spots and we get about 2-3 bars at both my house and her house.