Hello,
The device is indeed capable to connect to 5G. How the device makes the decision to connect to a certain type of network, is determined by the modem itself.
What firmware version is your device running?
Also, are you using automatically obtained APN setting or a custom value? In the latter case, is that the correct APN? Auto APN can be configured in router's WebUI Network -> Interfaces -> General page, by editing mobile interface settings, disabling Auto APN slider button and, selecting APN option value as --Custom-- and entering APN according to your provider.
Keep in mind that setting Network type to 5G only, will attempt to connect to 5G SA networks, which, at the moment are still being developed and deployed and may not be available with your operator. However, you could set it to 4G+5G, the device will attempt to connect to 5G NSA, which uses 4G infrastructure for its backbone, but will also be able to establish 4G connection.
Moreover, would it be possible to get a troubleshoot file from the device, after it is rebooted, but connects to 4G? To generate the file, access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file might provide more insight into your case.
Best regards,