Most traffic nowadays is HTTPS so as long as you configure your router to use a non-ISP DNS resolver such 1.1.1.1 (Cloudflare) or 8.8.8.8 (Google) then your ISP cannot see your traffic.

However, those ISP branded modem/router devices are completely backdoored and can be accessed by ISP employees for remote support. As they are your router they also get to see your internal network traffic. HTTPS traffic remains encrypted of course, but I personally would never let an ISP have access to my hardware.