My advice to anyone considering this, unless you specifically want the Expressvpn user interface, which is to be fair not too bad, don’t bother. The functions you lose outweigh the benefit of the gui (in my humble opinion).
Plus, getting back to the uboot interface was a ball-ache because the ip addresses changed (to be expected) but so did the reset button/led combination.
But just incase you try.
Follow the instructions from here:-
Uboot on AX1800 does not work! (post 22)
But use :-
Static ip to 192.168.142.66
Third led flash (blue)
Then
192.168.142.1