Wifi Gateway Authentication

Long story short; I have fixed wireless. About every month or so since I had the service installed, my wifi gateway seems to "forget" that I've authenticated my account with it. All connections will are still good, all lights on the router will be green. Resetting router and antenna don't do anything. Factory resetting the router doesn't do anything. What's happening is that when one of my devices makes a connection to the wifi network, it should be prompting me upon that first connection to enter my ATT ID & password to "authenticate" the gateway. Until this authentication happens, the gateway is blocking all traffic. 

The problem I have is that the only devices we have to connect are two different iPhones (a 7 and an 8-plus). iOS is blocking that authentication page because it doesn't trust the security certificate that accompanies it. The only other devices we have are a Roku TV and a PS4; neither of which deliver the authentication prompt.

ATT tech support over the phone is capable of fixing the problem over the phone, but it sort of depends on the tech you talk to. It's never been less than a 45 minute process, and in one circumstance it took a week because the phone support people insisted that I HAD to have a tech come to my house. The tech who came to my house apologized profusely that my time was wasted, but didn't offer any solution to fixing the problem myself. He just fixed it and left. Other times I've described in detail what the problem is to techs on the phone, told them to please not waste time going through their whole script, etc only to have them do so anyway and 45 minutes later be told "it turns out you were correct in your assessment, sir - the issue was that the authentication process had not been completed on your device. I've pushed that through so everything should work now." No joke. 

I'm here because the issue is happening again. I've already called tech support, only to be placed on hold for 30 minutes and hung up on.

Any ideas on how I can fix this myself if/when it occurs again? And any ideas on what's causing it to "forget" its authentication in the first place so maybe this can be avoided?