Internet connection issues to online game servers. (PC)

I switched to Xfinity recently. Using the Gateway modem and ethernet cable,  Even used a new ethernet cable. Still having the issue of connecting to online game servers. games such as  League, and other online Steam games.
it will sometimes connect but will disconnect within 5 minutes. 
have tried restarting the Gateway modem, computer, and reinstalling the game. 

Ty for any help