@JohnLai  Now the router totally can't connect to internet.Although it show online at status,but its can;t online either lan or wifi and it cause my whole wifi down once connected to my network as a AP.

@SSL_LEE 

Try to do a full reset and reconfigure the router from the scratch again.

Just to ensure previous settings are totally gone.

I assume you know how to setup the router for your ISP?

EDIT:

Do provide details on your network map. How does the router being connected, linked etc. You can't expect readers to know your network configuration without writing it down here. Your ISP information, fibre/DSL, ONU etc?

@JohnLai i already do reset once and it only work for 1 day. Previously work for 1 month no problem.i also upgraded to the newest version Archer AX50(US)_V1_210730 . This device act as a AP that connected to switch which is already connect to TM unifi modem and dial up router. As a AP, it is plug and play. Once connect to any network switch. It can be online already. Anyhow, i already try few times once connect to my network it will hang the wifi, i will proceed to claim.

@SSL_LEE 

Hmm, AX50 connected to a switch?

Just a quick question, have you disable all the downstream DHCP servers?

Might look like a broadcast storm or multiple DHCP servers issue. Try disabling DHCP server at AX50.

I suspect it might be DHCP issue because you mentioned it works for 1 day.

By the way, the connection from the switch is connected to AX50 WAN port or one of the LAN port?

@JohnLai switch as i mean is already connected to internet.i already off dhcp on the device.the device work for 1 day is after 1st reset. previously work for 1 month. definitely connect to LAN.