Volume buttons not working in fastboot mode (LG Leon H320)
I have almost the identical problem on a Rook phone from EE, which I understand is a carrier-rebranded ZTE Blade III A410. I  found this post by jbdubbs which might offer a fix, but I haven't tried it yet. Quote:


  When booting to fastboot with any sort of USB plugged in, your buttons on the side may not respond. In this case, pull the cable and hold down power + Vol Down until it reboots. The buttons should now respond and you can plug USB back in if you wish.


You can take a look at the original. There are a few other similar bugs described there.