Archer C20i using AC mode doesn't fall back for N clients

I successfully setup the Archer C20i and it seems to work on my wireless client if I use N mode.

But something strange happens if I use AC mode some of my N only clients will get "no internet"

This can be fixed if I switch the AP back to using N mode.

Does AC automatically fallback to N on OpenWRT?

Is there something strange a bug maybe on the mt7620 regarding this fallback behavior?

What's weird is the N only devices will associate and get an IP fine but then will fail with "no internet"

Anyone else know of this or a similar issue? Is this a flaw with the hardware on the Archer C20i v1?

Edit: It appears this is some sort of compatibility problem the device in question is an android phone the OnePlus5 to be exact when connecting to the archer C20i it appears this phone will get stuck on no internet indefinitely even though it supports AC. I tested another laptop that is AX capable and it doesn't seem to have an issue.

Not sure if there's a flag or something that is part of a feature set that is not supported which is causing this.