@RobustMeraki the MR76 is 802.3af compatible which means you can use any POE injector that is 802.3af compatible.
I believe the MG21e supports both 802.3af POE and DC power input. Once again using any compatible 802.3af POE injector is fine but I would avoid using 3rd party DC power packs, some cheap DC power packs are not true to their advertised voltage or amperage.