Here, I made a patch to force USB2. It's very basic and can't be configured. Someone more experienced should mainline this.
How to, by building your own image:
- Follow https://openwrt.org/docs/guide-user/additional-software/beginners-build-guide
- Add the .patch in openwrt/target/linux/ramips/patches-4.14/
https://framadrop.org/r/TSBEYUBp-1#8SwUOXRF2MXzJfzmnx/43O9M2IPWKH/0SPD604V+J9I= - Run make again
- Flash the sysupgrade file
- Enjoy !
(Please see the link for how to build a stable release vs snapshot, snapshots have no UI by default)
USB and 2.4Ghz works fine for me now !
Thanks to LGA1150 for the detailed info 