Thanks fmarcoux96. The Firmware Server Path was wrong. After correcting that the device upgraded to 22.214.171.124 easily (except for Boot firmware, which stayed at 126.96.36.199), see below.
However, the up-down-left-down arrow key didn’t work correctly - it wouldn’t go down, only up. So I thought I’d do a factory reset to straighten things out and start again.
Trying a factory reset from the Web GUI didn’t work. The device twirled for two hours, without achieving the reset. Why didn’t it work?
So then I did a factory reset from the device itself following the instructions in the Administration Guide 188.8.131.52. This went to completion, but now the WP810 is not functioning properly. The arrow key still won’t go up, but in addition many of the menu items are no longer selectable, they can’t be reached using the arrow key.
For example, I can see the WiFi network that I want to connect to, but I can’t select it. The arrow key is stuck on the first WiFi network (called wp_master) and won’t move to the WiFi lower down the list that I need to use in order to connect the Web GUI to the device. Therefore I can’t connect the Web GUI to the device to complete set up.
I’ve tried factory resetting again, and restarting the WiFi, but nothing works.
What can I do please?