Or upgrade them to modern OpenWrt, which has supported the near identical (minus USB port) D-Link DIR-882 since 2021: https://openwrt.org/toh/d-link/dir-882_a1 , as User 6SixTy has hinted at.
I actually have put that firmware on a MR2600, and it boots, not so sure about working well though. Screwing with OpenWRT and a vague sense it has a twin were my only memories of it. I had to retrace my own research from 4 years ago to come up with something worthy of a comment.