It has also been reported that the dongles can have difficulty picking up the signal when buried behind the Humax and all the wires. Have you tried it in the front socket? Alternatively put it on an extension USB lead.
Whichever one of the compatible dongles you buy it will contain the same chipset and very likely the same assembly rebranded. You might as well get a straight replacement as go to the hassle of a refund and another purchase.
I had to get a replacement on a WiFi access point which was dead on arrival, all went smoothly and the replacement was fine - it all depends on the attitude of the supplier, but they don't want negative comments so you should be OK.
I got an Edimax EW-7711USN via Amazon.
This version has a detachable aerial - which I utilised by plugging an extension to make it more visible to my wifi services.
I note that it is wide enough that I cannot get the HDMI socket in use, so have it on a short USB extension anyway (please don't point out the futility of this - I started with SCART rather than HDMI).
I had problems getting the wireless set up - kept losing it. After a little while I found that things seemed to be kept if I used a static IP address.
Without the custom firmware your only option (apart from USB drive) is FTP, although there is a method for download by DLNA HERE (click). FTP downloads are encrypted and of limited usefulness. FTP uploads are fine.