Good afternoon @becca2 and welcome to the Community!
Frustrating sure best describes your experience and 5 months is too long for persistent issues. May I know what iPhone model do you have? Have you had a chance to speak to our wireless technical support team? Based on what you've mentioned seems like you only started experiencing these data issues on your current mobile device. I know you mentioned that restarting the phone helps, but have you tried to reset the network settings to see if that helps? Just as an FYI, if you do reset the Network Settings you'll have to save your WiFi info. all over again. Also, may seem like a silly question, but are you on the most updated OS?