Welcome!If this modem has a built in router, it's best to bridge the modem. Having 2 routers on the same line can cause connection problems.
To tell if the modem is bridged or not, look at the routers web page, Status/Device Info/Wan Section, if there is a 192.168.0.# address in the WAN IP address field, then the modem is not bridged.
If the modem can't be bridged then see if the modem has a DMZ option and input the IP address the router gets from the modem and put that into the modems DMZ.
Maybe someone can review your router settings with you using
teamviewer if your interested. Its safe and secure.