I hope you get your problem resolved. I can confirm that the Velop Mesh nodes work well in such a setup. All they do is extend the network with wireless access points. I suspect you have a router or other misconfiguration problem that might still be there when you add the replacements.
When I used an Airport Extreme, I disabled NAT & DHCP on my ISP router and used the Airport Extreme as a router and firewall instead. When I replaced it with the Linksys Velop nodes, those functions (NAT & DHCP) had to be re-enabled on the ISP router, as the Velop nodes provide no such routing capabilities.