I have an AX1800 router connected to my Xfinity modem.  Works great, no problems.  
 

I have an Archer A20 in my study connected by Ethernet to the AX1800 router.  The A20 runs in access point (AP) mode.  
 

Until a few days ago, all connections to the AP were  great both through wifi and wired. But starting a couple of days ago ONLY wired connection works. That is, if I try to connect to the AP over wifi (tried both computer and tablet), there's no internet connection--appears that no IP address is assigned by the router. But, with a computer connected by Ethernet to the AP, internet access is fine--I get 400Mbps on speedtest. 
 

The front panel internet light of the AP is also yellow. 
 

What I've tried so far:

• factory reset of the access point
• replacing the Archer A20 access point with an A7 and still same behavior.
• "Forget the network" and try connecting again, no luck. 
• I've tried static vs dynamic IP addressing for the AP to no avail.
   

What could be going on with the router? The DHCP server can't have run out of addresses  since there're about 30 odd devices connected.   I'm at a total loss.  
 

Any help greatly appreciated.  Thank you.