Simple 3G/4G connection *TO* BBB, what's available?

I want to be able to connect to a remote BBB which doesn't have
reliable WiFi available. The obvious approach would seem to be a
3G/4G USB dongle and a SIM with a data allowance.

However I can't seem to find any information about using a mobile data
dongle for *incoming* connections, can anyone point me at anything
useful. I either need a cape that does it all or some information
about how to run programs that will 'listen' for an incoming
connection to a dongle.

Of course that also needs a matching 'outgoing' connection software
for my desktop or laptop (both running xubuntu Linux) but I think that
might be less difficult to find.

Take it, you’ve already stumbled across something like this or ruled out a tracfone as a mobile hotspot?

I'm a bit confused as to why a peripheral would want to establish an inbound connection to its host?

Normally, the dongle is just forwarding packets backwards and forwards in a largely stateless manner, and things like TCP connections are "the host's problem".

Making an inbound connection from another host on the Internet to the 4G-connected host is simply a matter of the 4G-connected host having a publicly routed IP address known to the connecting party at the time of the connection.

If the 4G dongle only provides a private IP address, then you'll need to configure the NAT gateway in between to forward the connection.

If that NAT gateway is outside your control (e.g. at your ISP), then your only option is to "tunnel" the connection by having the 4G-connected host establish an outbound connection to some publicly accessible point (*in* your control) which can forward the traffic down the tunnel to your host.

That is almost inevitably going to be the situation. However as I
already tunnel out through the WiFi connection I currently use for
this (which the 3g/4G will replace) it's not a big problem.