had same problem with speed declining significantly at night in rural area. Good in the morning till about 5pm when it drops to 2 mpbs. After several months with tect support and two trouble tickets. The second trouble ticket said the tower had too many customers on it at night and was over subscribed. So I returned the gateway and got my money back after three months for paying $50/month for a tower did not have capacity for the customers on it.
I had same problem 5g gateway had almost no speed but 5g speed on phone was fine. Tech said 5g phone signal and 5g internet signal are two different signals
Phones get higher priority... can really feel it in times of high utilization.
Expect to see more of it during March Madness.