Hi LokiUK,
I'm glad to see you got the dongle working securely in another piece of equipment. With it working elsewhere you may want to see if there are any firmware updates for the router and/or dongle.
Years ago I tried to get a network card to work, tried everything, drivers, different machines etc. pulled my hair out for a few hours, nothing would get it to work and it made no sense. Left it disconnected overnight, went back to it the next day and it worked first time, I hadn't done anything different to the previous day. The card was just retaining a messed up configuration even between machine swaps and it just took longer than I expected for the circuits to discharge. Sometimes these things happen.
My advice, remove the dongle, delete all of the network settings in the Humax, switch it off at the back, plug the dongle back in, switch the Humax back on again and power it up, wait a few minutes and try again with the network configuration.
If this doesn't work and you have to have wireless then I guess the only option is what you previously mentioned which would be the powerline/homeplug system.
good luck with it and let us know how you get on.