Id factory reset and test again.
- WAN Port Speed set to Auto or specific speed? Some newer ISP modems support 1000Mb so manually setting to Gb speeds can be supported by the router. Advanced/Advanced Networking/WAN Port Speed
Cat6 cables are recommended for use. I've been using CAT6 for years now, short and long cables. No issues. I'd swap the cable between the router and modem. I don't use inbox cables as I've seen bad ones come in the box.
Factory reset, resend the same FW version or update to v1.02, Factory reset once more then set up from scratch.
Try this router at a friend or families place to see if the problem follows, then you'll know if it's a problem on the router or else where.
I have this same modem model and have never seen this issue.