Plug the mesh node via its WAN port. I think using LAN is causing it to behave as if you are creating a wireless bridge.
Out of the three original units only one is left that has 3.0.34 firmware but when I swapped that firmware to 1.0.44 (chinese version) it did allow mesh from AP mode.. whereas the international firmware only allowed mesh from full router mode.
Tell me what firmware you are running on AP unit and mesh node unit and I can run a test to see if it works.
The whole of this thread has the topic of how to get SSH access.. sorry but you need to read it. There is no single tutorial site unless you use Chinese firmware and once there you cannot go back.. but it is easy from 1.0.17.
See the youtube link near the top.
The only way to get ssh access with international firmware is using the second true OpenWRT router... You can probably pickup something second hand for a few dollars.
I followed this exactly.. It is for the Redmi AX6 but the AX5 is close enough to work. Don't expect instant success .. it takes loads of patience to get the sequence exactly right. At each step you must get the right response from your browser connection.. otherwise go back and start over.. which means every time you will get a new stok key.
Note a computer with both ethernet and wifi network adapters makes life a lot easier.
That gives you SSH but it is not permanent. It will survive reboots but not reset, firmware update etc.
So the second bit means you need to follow the link I gave in the second post to Oded.dev and here things start getting complicated because a key part of his method simply did not work. If you get the first part done post back and I can perhaps help a bit with the second bit.
Note that this led to me having to flash Chinese firmware as it also lost the password in two of the units.