Wireless router (mode) PC cannot ping AP (mode) PC on 2nd router

[wistar38]

For ASUS RT-AC66U (2 routers, up-to-date firmware):
Router 1 (wireless router connected to cable modem) with Ethernet connection to Router 2 in AP mode 75 feet away.
PC on Router 1 cannot ping IP for PC on Router 2.
PC on Router 2 can ping IP for PC on Router 1.
PC to PC pings fine on Router 1 to Router 1.
Setups checked multiple times. No duplicate IPs or MACs.
Any thoughts?

 

[Tech9]

Sure Router 2 is in AP Mode? All devices on the same subnet?