arjendekorte:

You can only use 40 MHz in the 2.4 GHz band if there aren't other Bluetooth, microwave or Wi-Fi emissions in the neighborhood by using two adjacent 20 MHz channels. This is unlikely in residential areas nowadays. Older equipment may do so anyway or allow you to override this restriction, but this will not help you to increase throughput if the adjacent channels are indeed in use.

Aye24:

With the stock netgear firmware it's always at 40MHz, and I don't have other wifi near my house.

IronicSven:

Do you see something like "20/40 MHz operation not permitted on channel XXX based on overlapping BSSes" in the log?

Aye24:

No, I don't have logs related to this problem.

IronicSven:

I've meant "Status" > "System Log".

You can force 40 Mhz with the noscan option: https://wiki.openwrt.org/doc/uci/wireless

But keep in mind that turning this on might violate regulatory requirements.

Aye24:

Yes in "Status" > "System Log".

With noscan it works, thanks a lot.