4G modem >OPNSense > ZeroTier can this work?

[jaimito]

Hi

we have commercial support for our move to OPNSense but this concerns my own setup.
 
When in rural Spain I no longer have access to WiMAX which although slow gave a fixed IP and decent IPSec to London (as Telefonica have dumped WiMAX altogether).

I now only get connection via 4G with dynamic IP changing somewhat  frequently, so IPSec not feasible.

What I hope to do is to use ZeroTier to establish a VPN connection to London so that I can run VoIP over it to my office Asterisk exchange.

I will be using the provider-supplied 4G modem/router which has an ethernet port.

So a schematic would be:

^
|
--[4G modem]--ethernet--[OPNSense running ZeroTier routing VoIP]--switch--clients including VoIP phone (Siemens)

My questions are concerning how best to get this to work. I assume double-NAT would be a bad idea both for VoIP and ZeroTier -- the 4G modem/router by default does NAT, and I have not had much success in setting it to pass all traffic to a designated DMZ, port opening may work though.

-- will this idea work at all?

-- should I set the OPNSense to no-NAT?

-- or forward ports to it?

-- has anyone done anything similar and got it to work?

Any clues or suggestions welcome -- I am in the UK at present but will be on-site there later in the year.

Thanks in anticipation...

MeJ