Thanks for that 
The mt7981b-openwrt-one.dts has the following:
#include "mt7981.dtsi"
But the upstream kernel doesn't have that .dtsi file, but it does have mt7981b.dtsi and the Hardware Specs do say "SOC: MediaTek MT7981B".
Does this mean that the #include is incorrect and it should be mt7981b.dtsi?
The Hardware Specs also have this:
"Wi-Fi: MediaTek MT7976C (2x2 2.4 GHz + 3x3/2x2 + zero-wait DFS 5Ghz)"
I'm not an expert in DeviceTree, but not clueless either, but AFAICT the wifi is not (yet) defined in the dts[i]?
Grep-ing through the kernel source for "MT7976" (only) returns results in drivers/net/wireless/mediatek/mt76/mt7915/. The Kconfig file only has MT7915E and MT798X_WMAC, but their description don't seem to match the HW Specs?
So what kernel modules need to be enabled to add support for Wi-Fi?