even a wireless connection is possible

That's kind of broken. You won't be able to connect and maintain stable connection, because the wireless firmware fails to start on Linux kernel 5 and newer. I plan to put my changes on the latest version of OpenWrt 19.07 to see if the firmware works on kernel 4.14.x

In the meantime, I will update the current fork to the latest master commit as you requested. There's a change on upstream that conflicts with one of my patches. I'm working on solving it.

I flashed the trx file via the router's CFE interface

Did putting the OpenWrt trx image on the CFE web UI work? I had to run a TFTP server and a command like http://192.168.1.1/do.htm?cmd=flash+-noheader+192.168.1.2:firmware.trx+flash0.trx on the browser to bypass the image integrity check.

Edit: That conflict is a bigger problem than I thought. I'll work on it in the upcoming days.