Some troubleshooting questions from a networking point of view (I don't have an Omni panel):
- Can you confirm you have the proper subnet masks on every interface on every device?
- Are you using any Jumbo frames anywhere?
- Is UPnP disabled on the GL-iNet router (in general, this really should be disabled everywhere)?
- Does rebooting just the GL-iNet router fix the issue as well?
- How old is the modem? I've seen some weird behavior with older DOCSIS 2.0 modems
- Do you see any errors on the LAN or WAN interface on the GL-iNet router using ifconfig -a, netstat -i or netstat -s? Feel free to share the interface counters. I've seen similar behaviors in the past due to firmware/OS bugs where large values may cause this type of degradation.
- If you have a spare switch, I'd consider plugging it into the LAN side of the GL-iNet router, plug the Omni into the switch, and check if you see this same behavior when pinging from a test machine plugged into that switch.