It may not be the router at all. My AT&T goes through spells of dropping the signal but then it will run for months without an issue. I suspect occasional power outages or other problems at the tower is the culprit.
After one of the hurricanes that hit our area we were having no signal from our Verizon phones. Turned out people were stealing the generators from the towers. Not sure why they don't have all the towers fenced like they do with the police band towers. Rumor has it they assigned National Guards to the cell towers to protect the generators until power could be restored.
BTW, they will NEVER admit when they have a tower issue. IT NEVER HAPPENS!