Linksys 1900ACS v2 (EU) keeps rebooting after being online for a few minutes

Hey guys,
Currently trying to use the modem Wifi for wireless and i'm having issues, After some problems with the version 19.07
I went back to the latest 18th version and everything sems to work good so far the wireless connectivity and speed is pretty good but one thing i don't understand is that the modem keeps reeboting for some reason and comes back online is there something i need to know or do to prevent this?
Thanks for your help.

As far as I know there is a package called watchcat which can do periodic reboot when connection is down or every X minutes. Could it be that? Or any other cron based reboot? Is power supply delivering the volts and amperes it should?

The WRT1900ACS v2 does not have a modem built in, therefore: what modem are you talking about?

He means the router.