Have you tried a different cable?
Do all LAN ports of RUTX50 result in the same speed?
I would like you to check, if the following results in some improvement. Login to router's WebUI, navigate to Network -> Firewall -> General settings and enable Software flow offloading.
Also, could you login to your router via SSH and execute the following commands:
- swconfig dev switch0 show
- ethtool eth0
Then paste their output in a some text file and attach it by editing your question.
In addition, could you provide some screenshots of speed tests?
Lastly, I would like you to attach a troubleshoot file to your question. Please, replicate the issue, then 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 the issue.