Changing this counter value from 100 (25seconds) to 200 (50 seconds) works for me:
https://github.com/vyos/vyos-1x/blob/f6a1ff94f02282bd018b9b49ae1a9812619ec7ba/src/conf_mode/interfaces_wwan.py#L143

Running Commit again after the modem 0 is exist does work.

I also ran into an issue where isc-dhcp-relay could not be made to work in a DMVPN setup. Using dhcp-helper does work, but I would like to point out that in its current state, dhcp-helper binds to UDP 0.0.0.0:67 even when you specify a particular interface.
This causes problems with isc-kea-dhcp4-server, which attempts to bind to a specific interface (UDP <interface IP>:67). However, this is not allowed because the wildcard binding (0.0.0.0:67) already exists.