Netgear RBx50 - Using a Channel under 100 for QCA9984

I have 2 RBX50's as satellites connecting back to a main router. They use the QCA9984 wifi for the backhaul connection which works fine. However I want to use a channel on my main router lower than 100 but the RBX50's are not able to select a channel any lower within LuCi.
Does anyone know whether this is this something I can just set using the command line and override what is available in via LuCi, or is it something hardcoded into the firmware of the device?

it's a deliberate limit set by the vendor.

Is there anyway for openwrt to set it to a lower channel, surely the QCA9984 is capable of using those channels or have they hardwired something?

It's a physical limitation.

The device has 2 5G radios, one is using the upper bands and the other one the lower bands.

Usually the radio frontends are having HF filters to make sure, that one radio is not interfering the other one.

So even if you could configure the radio to lower channels, it won't work due to the frontend filters.