Sorry, it seems this bug has nothing to do with router as FTP/SAMBA server. Set LAN ip address and pool range, and then set wifi radio could cause it even without an USB device presented.
FYI, ASUS's own 3.0.0.4.384.10290 and 3.0.0.4.386.45934(latest stable from ASUS) also has this bug, you...
Affected model: AX56U
Affected Merlin version: 386.2_6 through 386.5_2
How to recreate it: after copy large files from FTP/SAMBA server on the router, switch on and off WIFI 2.4G radio several times, all LAN port will be dead.
Additional Info: After all LAN port are dead, restart the router...