@RkRx,
Are you able to log into the router interface and check what the negotiated link speed is?
At the bottom of the network option under the advanced tab you can find this setting:

If it is set to Auto Negotiate, try forcing the connection to 1000Mbps Full Duplex. If its maxing at 100, it most likely auto-negotiated to 100Mbps full duplex.
If your router is unable to connect to the modem, then I would recommend trying a different cable to connect.
Lastly, since the 2.5Gbps port is a WAN/LAN combo port, you may try connecting a PC(if it can support multigig or even gigabit) to the port and seeing what the negotiated speed is on the side of the PC is. The speed can be found in your network adapter settings, usually under connection properties and it will be labeled as link speed.