ZBT WG3526 4G to LAN doesn't work

Well when I wanted to load the downloadable version here it crashed the router! I had to ask the supplier for the original firmware to reload it via the recovery procedure with the bootloader integrated into the device !!!

To be fair, though, you forced the upgrade. The vendor firmware seems to be based on lede (17.01) which is very old (6 years, 4 major versions). Many things have changed making a direct upgrade very unlikely to work in any scenario. Upgrading first to openwrt 18.06 and then to either 19.07 or 21.02 would get you into a position to upgrade the rest of the way to 22.03.