Should DMVPN work over the MG21? I have got it to work and stay up all night. But to test stability, I shut/no-shut the tunnel interface on the router and DMVPN will not come back up. At some random point through the day, it may come back up. But I've never had this much trouble with DMVPN before and am beginning to wonder if the MG21 is denying access in.
I think you'll need to be using at least DMVPN phase 3 or better to make this work (which you probably are if this is a recent DMVPN implementation). I would make sure I am using the "ip nhrp registration no-unique" option on your spoke tunnel. If you aren't using this option yet then it doesn't take affect until the existing NHRP registration on the hub expires.