It is not quite clear if you simply want between the RUT devices or access to the network behind the RUT.
Regardless, there are several options you can go with for remote access. Be aware, that most of the require at least one RUT to have a public IP address.
Remote HTTP(S), SSH access.
This is the least recommended option, due to security reasons. You can enable remote device access from System -> Administration -> Access control section, but before doing that, replace standard port numbers with any random number above 1024.
The simplest solution for remote local network device access. It can be configured in the router's WebUI Network -> Firewall -> Port forwards section. There you have to specify the external port, which will then be redirected to the associated local device and service port you want to access. You can find more details about configuration options here. Keep in mind that port forwarding is not the most secure solution too and for this I would suggest to make sure that you use strong passwords on the devices you enable access to, if possible, restrict access by source MAC, IP addresses, set unconventional destination port.
uPnP (Universal Plug & Play).
This is a service which allows to map internal devices and ports for external access automatically and it seems that PLEX does support it. For this an additional package might be needed to download in the router from Services -> Package manager. For additional configuration details, please refer to this page.
This solution provides more security. The go to options would be OpenVPN or Wireguard, however, Teltonika supports a number of VPN protocols. For the reference configurations, check the following links:
OpenVPN link. Instructions here are made for legacy design firmware, but apply for current RutOS as well.
Zerotier link. This VPN has an advantage in that it does not require a public IP address.
Since these are Layer3 protocols, the LAN network you initiate the access from will have to be in a different subnet than the LAN network you want to access. As an alternative, to bridge the networks, you can use TAP mode of OpenVPN instead of TUN, but you will need to be mindful about the devices IPs on both networks.
Teltonika also provides it's own cloud solution, Remote Management System, which allows to access, monitor and manage both, the router and the devices connected to the routers. The feature to allow remote access to your controller that I would suggest is VPN HUBs. You can see how RMS is configured to enable remote access in this video.