OK, that should eliminate power issues as the problem.

Reviews would seem to indicate that it could be a modem issue.

Possibly useful links:

https://www.windowscentral.com/best-docsis-3-1-modems

https://www.pcguide.com/modem/guide/best-docsis-3-1/

https://networkshardware.com/modems/best/docsis-3-1/

https://www.amazon.com/NETGEAR-downl.../dp/B01I5TJGSE
(check the reviews)

https://www.amazon.com/MOTOROLA-Appr.../dp/B0723599RQ
(check the reviews)