Okay so, fundamentally all the issues I have been having appear to be linked to the fact the router:
Xiaomi AIoT Router AX3600
Has a Qualcomm chipset.
On the stock firmware, only the Qualcomm Atheros QCA9887 802.11ac/b/g/n has the required ap/vlan option and this is limited to 802.11n even when 802.11ac is available.
Other NSS firmware don't seem to have this patch, and when tested fail.
Found this:
Created a build enviroment and in the process of compiling. When attempting to do so with the NSS firmware of the 3 I found, they never complete compiling. So, will just start with the basic stock firmware and level up from there.
If anyone is wanting to know, this is for Xiaomi AX3600 but should work for amy ath11K chipset router.
Will post an update once I have a successfully compiled firmware to test.