Grinding when shifting into the AX-15's or AX-5's non-synchronized Reverse gear is normal unless you first momentarily shift into a forward gear first before shifting into Reverse. Try that.
And since it's getting cold where you are, maybe changing the gear lube to a good synthetic would help. #1 favorite gear lube for the AX-15 and AX-5 transmissions is Redline MT-90 which you can get fairly cheaply from www.summitracing.com
. Synthetic transmission lubes provide better shifting characteristics in cold weather.
Fresh gear lube is not a miracle cure but it can't hurt at this point.