@Amds You can't use the internal DNS server on for the WAN Port DNS. You enter the Local DNS Server on the VLAN DNS section.

@haswalt Change your WAN dns to google and or cloudfare. See if that helps? Confirming the ISP is solid?

@TimothyLambert Also considering going to Settings>Transmission>NAT>ALG. Disable all the ALG's and retest.

@TimothyLambert The best way to look at it is... Click on the client list and click on the 3 dots on the right. Check customized columns, check signal, wifi connection, rx, and tx. This will show you...

@TimothyLambert 2 things here... What DNS Servers are you using? Use CloudFare or Google DNS. Also by chance, do you know the signal strength of the clients that are having issues?

@TonL Have you tried a new cable between the cable modem and the WAN port of the router?