Hello Andy,
have a look at the following thread. It refers to use of eBGP in DMVPN phase 3.
https://community.cisco.com/t5/routing/ebgp-and-dmvpn/m-p/3754562
If you want use eBGP with DMVPN phase 2 the hub router may need some tweak in BGP configuration to avoid to rewrite the BGP next-hop attribute (the opposite of default behaviour in eBGP) or you could use iBGP and make the Hub the route reflector servers for all the spokes.
Edit:
the command is neighbor next-hop unchanged but it is supported only on eBGP multihop sessions. ie you should build eBGP sessions over loopbacks. this is for eBGP in DMVPN phase 2 to avoid the hub to set the BGP next-hop to its own IP address that in phase 2 would block dynamic spoke to spoke tunnel setup.
see
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-16/irg-xe-16-book/irg-next-hop.html
Hope to help
Giuseppe
