@IrvSp Thanks for bringing up the UPS issue: I have a Tripp Lite UPS unit from the wall, which feeds both the modem and the tp-link router. My ISP was blaming the signal box outside the building, where I get the coaxial cable from. They said unstable signals (in frequencies) may force their modem to restart. Weeks ago, their on-site person was confident that he fixed the signal problem. He also assured me that "his signal box" is grounded, so I don't need to use the UPS unit to relay the cable. After all these (and a couple of months, or say, for a whole year living with random interruptions), last week, I was kicked out of my apartment 10 minutes before a scheduled meeting lol.

While the modem reboots on itself, I did have consistent access to the tp-link router. The router was a bit disappointed as there was no internet connection. All other units attached to the same UPS was running just fine. To further isolate the problems, let me add a separate UPS unit for the modem + router alone. Anyways, they deserve to run on battery power.

I am moving to a new apartment and will see if completely isolating the signal box issue may stop the service interruptions on the ISP's modem. (My landlord sold the place while I have a couple of months left on the lease. I was made to move to a building down the block, likely, running the coaxial cable from the same pole. If the interruption continues, I now have the squirrels to blame.)

@Linfeng 

i do NOT use the USP to filter out the cable from the wall. Only the Electric power.

I've been told never to do that, even with a surge protector that has that feature. It can degrade the service. Besides, I do NOT own the modem, it could be damaged, but the ISP will replace it.