This might be an issue with your main router (Peplink), to rule this out could you set your router to bridge mode and plug the ethernet cable into the router LAN port and another end to your PC then check what IP address you are getting? Regular DHCP or your ISP-provided IP?
The IP address you provided is definitely from your ISP provider, as it falls within the 10.xx.xx.xx range, and masks like 255.255.255.252 are commonly used by internet providers.
I would suggest examining the settings of your other router to see if static IP or other configurations are set for the WAN port where the RUTX50 is connected
I’m not very familiar with this manufacturer’s firmware, so I can’t provide detailed information about what happens “under the hood” for each setting. However, at first glance, everything appears normal.
You might want to try adjusting the Connection method and Routing mode settings, as they could function differently for this router.