AC1200 router dropping Internet repeatedly

I have a AC1200 Smart WiFi Router, R6220, purchased in 2017 with current firmware version: V1.1.0.50_1.0.1 (no new firmware version available).

A couple weeks ago our Internet started going down repeatedly and we would have to power cycle the modem to get it back up. We obtained a brand new modem from our provider (Optimum) and they paid a visit to our home to install a brand new cable drop from the pole. However, after all that our Internet keeps going out every few hours. I have found that simply unplugging the ethernet cable from the modem to the router and plugging it in again immediately solves the problem, without power cycling anything, but this only solves the problem for another couple hours when the Internet inevitably goes down again. We have nothing plugged in to the router except for the ethernet cable from the modem. I see that this model router is still being made so it doesn't make sense to just purchase a new one. Please advise on how to solve this issue. Many thanks!