Adding kmod-usb-dwc3-of-simple back in didn't fix anything unfortunately.
From what I can tell it works on some target platforms and not others (ipq80xx and ramips work, while ipq40xx, rockchip, and mvebu do not*) on 21.02 builds, and additionally it may have been fixed in master according to the second thread linked below, although I haven't even tried to find the relevant commit(s).
*I don't know if this can be said/is true for all sub-targets of each respective target
Edit:
I searched for commits in master (I'm guessing that's what @richardhd from the second thread I linked was building from) that referenced any of the things involved in iOS tethering between the dates of 2-20-2021 and 2-23-2021 (or at least I think I did?) and one seems like it might be worth checking out https://github.com/openwrt/openwrt/commit/4b37e3bc2b2a079c996b6d97b8d3dbbd4ba6eb62.