Hello,
In that case, I would recommend reverting the previous command using this one:
ethtool -s eth1 duplex full speed 1000 autoneg on
To set the port to 1Gbps speed.
As for the CGNAT address, I wouldn't imagine it causing any issues, as many of our devices use SIM cards that provide a CGNAT address. To be safe, try setting the LAN IP of the RUTX to 192.168.5.0/24 to ensure this is not an IP conflict.
Perhaps the settings of the Starlink router indicate some differences in configuration?
Could you try setting the MTU to something lower? 1200, 1300, 1400, etc.
Best regards,
DaumantasG