I just used an example from Teltonika's Wiki. You can find it here.
My config looks like this:
Notice that remote/local tunnel endpoints are inverted on each end. Also, don't forget to specify IP addresses in accordance with your own configuration. Instructions on how to generate a static key can be found in the link that I added or with an online search.
Should be pretty simple. When I tested it earlier, the router had trouble sending TLS certificates via SMS. This way it only has to send the static key. I tried it a few times and it always worked properly. Hope it works out for you as well.