TP-Link C2600: firmware update broke 2.4 GHz radio

I run stable OpenWrt (currently 19.07.6) on a TP-Link Archer C2600. I've generally used standard firmware images downloaded from the OpenWrt website, but have recently started building my own, in order to include some software I use (iperf3, ddns) that isn't included in the standard firmware (i.e., to avoid having to reinstall it each time I upgrade the firmware).
This has been working fine for me, until yesterday, when I built a new firmware to include SQM. The only thing I added to my previously working image was luci-app-sqm, plus whatever dependencies it brought in. Since the router came up with this firmware, the 2.4 radio (radio1) has been stuck on disabled ("up": false, "pending": false), and simply will not enable itself and work, whatever I do. I cannot be certain that this is due to the inclusion of SQM, of course.
Any idea of how I can troubleshoot this? Obviously, I'll have to revert to other builds and see if the problem goes away, and then try installing SQM on a standard firmware, but is there any troubleshooting I should do before I start the tedious process of flashing, rebooting, and disrupting the household's internet?
The OpenWrt version is r11278-8055e38794

Do you have any USB devices connected, IIRC they may interfere with the 2.4GHz radio?

Yes, I saw that on various forum posts - no USB devices connected.

I reverted to OpenWrt built firmware, and installed SQM afterward, and 2.4 GHz is working now. I'm not sure whether / how to troubleshoot further.