There is a list of links to the dictionaries in
https://www.mobileread.com/forums/sh...d.php?t=196931. The English dictionary is still there. You could try downloading that and manually installing it using the instructions in the third or forth link. If the Mini isn't downloading these, it probably means the link the older firmware uses is no longer valid. That was probably an indirect link via Akamai or something. They probably only have the current dictionaries available in that way.
But, I agree with @PeterT. Firmware 4.28 works on the Mini and has some improvements. If you don't particularly want the homepage that 3.19 has, I would suggest using the later firmware.