Fiber internet goes out on sunny winter days

We have been having a chronic ATT Fiber internet problem where the internet will go down for 30 mins to 3+ hours on sunny winter days. This has been going on for 2 years now (Nov-March each year) and ATT has been unable to resolve it.

We have had a minimum of 6 tech visits to the house. The ONT box and inside cables have all been replaced. A new gateway router was provided by ATT. The problem is that nobody seems to think it is an outside problem, but it has to be due to the weather conditions that are the one constant in our problem. 

It does tend to go out at the same time of day on those sunny winter days. During this time the sun is hitting our green ATT box in our yard, as well as the side of the house where the fiber connection transfers into the house. 

Internet is out again today as we are experiencing a nice sunny, 30 degree day here in Michigan. A tech is coming out again tomorrow. (maybe if I have them send a tech out twice a week, something will get done) Does anybody have any suggestions on what I should tell them to try and get some better troubleshooting of this problem?

Is there any "escalated" tech issue phone number I can call?  I would love to talk to someone who is not just going through the "recommended" steps, but actually understands that this will take more to resolve. The techs that come out are absolutely against putting in an escalation. They have told me that they get "penalized" if they don't fix the problem themselves. They are all very nice, but at this point, unhelpful.

The whole thing is maddening. I love the ATT fiber service when it works, which is the only reason I have put up with this so long....but at the end of my patience on this issue.