I have a AX10 router working in router mode, and a RE305 range extender. 
The AX10 obtains the internet IP address normally - internet IP is in 192.168.0.0/24 subnet, and AX DHCP server serves the range 192.168.1.100-255. 
When the router starts up, everything seems to work fine. Every device connecting from either RE305 and AX10 gets an IP address.

After some time, new devices connecting in RE305 for the first time or reconnecting will not obtain an IP address because AX10 will not assign any. Devices connecting to AX10 directly get IP addresses assigned normally.

At first I thought this could be a RE305 issue, but I've also noticed that when this happens, the devices that are already connected to either RE305 and AX10 get routed to the internet normally, but the IP addresses in the AX10 LAN subnet cannot "talk" to each other. For example, I have a UDP based app that stream data from a smartphone to a server app in a PC, and this stops working. I try to ping between my PC and smartphone LAN IPs, but get destination unreachable messages (firewall is off in the PC).

When I reboot AX10, all goes back to normal for a while, until it stops again. I have no idea what could be triggering this issue, but it is pretty annoying.

I've tried resetting both AX10 and RE305 to factory defaults, but the issue remains.