If you are using an Ethernet PoE switch to power an Active to Passive PoE converter, then the Switch port needs to be 802.3at, in order to deliver the 18W potential maximum power consumption of the RUTX50. Needless to say, your PoE Converter has to also be capable of delivering this output or more.
This is all assuming that the RUTX50 is being powered through its LAN Port 1. If so, I’d also be investigating how well the RJ45 connectors are attached to the cable. Time to ‘shimmy’ up that mast, and whilst you’re there, check the water-tightness of the RJ45/cable to the device and enclosure.
Also if in a marine environment, then give the SIM a thin coating of dielectric grease and spray or ‘paint’ something like ACF-50 onto the connectors of plugs and sockets, including the RJ45’s and anything else that may be affected by a saline environment. Treat your PoE converter to the same inspections and ACF-50. This should become an annual maintenance task.
For that distance of cable run (15m), I think we can discount the AWG size of your ethernet cable.