Hi and welcome to SF,
Could you please try changing your DNS servers in your router to these Google Public DNS
Please do a re-boot on router.
Please power off modem.
Power off router.
Power off any computers or game consoles.
Leave for 2 minutes.
Power on modem and wait for lights to stabilise.
Power on router wait for lights to stabilise.
Power on probmatic computer login and wait until fully booted, then please check for internet connectivity and see if it is stable.
Please let us know the results.