ASUS Router Issue(s)

[ElBoBo]

I have an ASUS RT-AX86U router and my ISP provides 1Gbps up & down, but does not provide native ipv6. I get the full upload speeds, but only half of the download to the router. The issue seems to be somewhat related to ipv6. When ipv6 passthrough is enabled, every 0-2 seconds I get:

kernel: protocol 0000 is buggy, dev eth0

I'm using Cloudflare's ipv4 & ipv6 dns servers. The error ceases if ipv6 is disabled; however, when ipv6 is disabled, the download speeds are no better.

 

It's worth noting on my prior ISP with the same router, I had my full upload and download speeds. However, my full upload was 40Mbps, not 1Gbps like it is now. Another thread mentioned:

On 3/19/2018 at 1:18 AM, KarathKasun said:

Is this from a router log?  If so, its the router, possibly a chip or board design shortcoming.  Most routers are not designed with Gig uplink speeds in mind, so they just use "good enough" parts.

However, the error in that log is different from mine:

kernel: protocol 0800 is buggy, dev eth3

Keep in mind, my errors go away when ipv6 is disabled. Despite this, could these be the same issue? If so, is there a different router I can purchase that doesn't have that issue? If not, any ideas on what the issue is and/or how to fix it?