Using existing Linksys Wrt1900 AC as a router instead of AP?

Switch not router ***

Setup

-ISP modem

-Linksys WRT1900 AC with both radios turned off.

-UniFi AP connected to LAN using the port on Linksys

At first, speeds were great when I ran tests. Then all of a sudden there’s a “detecting ping error that could be caused by a firewall”

Would this be the built in firewall if the Linksys?

I’m waiting on the USG and a standard switch but wanted to test this thing out in the meantime

Any suggestions ?