First thing, can I live vicariously through your fast internet connection and I would like to try, if this is ok,
On your motherboard, you're using the realtek 2.5 (upper network connection) try the lower one which is the Intel I211-AT (2.5 as well) and retest / see if this changes speed at all in your current config.
If I were to suggest to you the 2.5 Gbps port was for LAN only and worked at 1 Gbps when in WAN mode, would that fit your situation..
On my system, the only way I can get above 1 Gbps is to use a 2.5 Gbps port on my computer and a Wi-Fi AX system which is local only. I don't have a high speed ISP so I cannot test.. You may see if you can find a router with an advertised 2.5 Gbps WAN port..