The Archer AX73 defaults to a WiFi country of DE.  I verified this with Wireshark doing a signal scan of the beacon frames.  I am in Thailand where the router was purchased.

Each country has regulations which govern the Radio Spectrum and in this case the WiFi spectrum, and what maximum signal strength is allowed, and what DFS (Radar Avoidance) algorithm must be employed and what Channels can be used.  We should also be able to set the WiFi to Indoor or Outdoor Environment, as different rules apply for Indoor and Outdoor use, currently its defaulting to "Unknown".

I can find no way to change the country code on the router.

To Operate the router in conformance with the regulations in Thailand, the router should bee set to TH and should obey the Rules set by the regulator for Thailand.  The Rules set for DE (Germany)/Unknown Environment are NOT the same as the rules allowed in Thailand in ANY Environment.

These are the German Rules:

country DE: DFS-ETSI
    (2400 - 2483.5 @ 40), (20db)
    (5150 - 5250 @ 80), (23db), NO-OUTDOOR, AUTO-BW, wmmrule=ETSI
    (5250 - 5350 @ 80), (20db), NO-OUTDOOR, DFS, AUTO-BW, wmmrule=ETSI
    (5470 - 5725 @ 160), (26db), DFS, wmmrule=ETSI
    # short range devices (ETSI EN 300 440-1)
    (5725 - 5875 @ 80), (13db)

These are the Thai Rules:

country TH: DFS-FCC
    (2402 - 2482 @ 40), (20db)
    (5170 - 5250 @ 80), (17db), AUTO-BW
    (5250 - 5330 @ 80), (24db), DFS, AUTO-BW
    (5490 - 5730 @ 160), (24db), DFS
    (5735 - 5835 @ 80), (30db)

For Example, as you can see above, in Thailand the WiFi is supported from 5735-5875 with a power of 30db,  DE restricts that range for low power and short range.

By not allowing me to change the country you are forcing me to run a router that is not conformant with local regulations.

How can I set the country code?  Please advise.

I also have an AX11000 which is sending US as its country code as confirmed today again with a wireshark trace of the Beacon frames.  Again, this is not conformant with Thai regulation, so I need to know how to set the country code for Thailand for that model router as well.

It would also appear that forcing the router to DE has artificially limited the available selection of 5Ghz channels, as I can select much fewer with this router than I have been able to with other routers when properly selecting TH as the country.