What exactly is "crashing" with you? You can set up the 4/5G modem to restart regularly. Otherwise, you can also set up a passthrough so that you have the IP of the mobile network directly on your target device. Alternatively, you could set up a DMZ. Since this is a gateway, I would recommend you to use a dedicated firewall behind it. As a router, I would recommend this device: Teltonika RUTX50 article number
22566042.

I use the device as a simple bridge, with a firewall behind it.
The device has been running continuously for weeks without any errors. Performance as on the first day.

I don't use port forwarding, but the TRB500 for internet access and as an openvpn client. openvpn config was difficult, but it runs stable with the Mikrotik CPE (openvpn server). It's my first Teltonika product and I'm satisfied so far. VPN config examples could be better described.

I do not use port forwarding, but as an openvpn client. This runs well and stable. However, the effort to configure opevpn with Mikrotik CPE (server) was difficult.

you have to ask for support from digirec. but they don't know what they are selling, so you have to contact teltonika directly - they don't have end user support, contact them via contact form in english. ask for the developer firmware version TRB5_T_H744_00.07.04.112 or newer. i don't know if it is already on their website for download. but yes, sending it back is also a good idea. huge hassle with the box.