It looks like (MartneauUK) has added the RT-AC86U to his Wireguard script, his stable (VERSION="v4.11") was posted earlier today.
See here: https://github.com/MartineauUK/wireguard/tree/a334177b494f520747aae6405782012b3948b2aa
also here...
Today I decided to upgrade to the 1.0.20210124 kernel for my RT-AX88U, when downloaded using "wget" followed by copying and pasting
the link from post number 1. When I tried to install it returned the same error you got, when I checked the size and compared to the one
I had downloaded on my...
Have you been able to download and install wireguar? If not try these links for the RT-AX86U.
ssh your_username@your_router_ip
Type: "wget" space (copy and past one links each time) on you terminal...