if you can't get mobile data to work, it might be because you left the checkbox for your sim card at its default unchecked value in the initial setup wizard. it's essential to check that box manually, even if you only have one sim card - at least on the oneplus5 it is.
unfortunately, if you didn't check that box, i don't think there's any way to resolve the problem via the UI, or at least i couldn't find one. the mobile data toggles simply don't do the trick.
on the other hand, with some comparisons of twrp backups, i was able to find the setting that controls this, and find a command you can run via adb to fix it, for sim cards in the first slot (your mileage may vary, no warranty implied, etc.):
Code:
adb shell settings put global multi_sim_data_call 1
(you can run the same on your device if you have root access but using "su -c" instead of "adb shell")
i assume you can change 1 to 2 if your data sim is in the second slot, but i haven't tested it.
if you don't have adb or root, i think you'll either need to wait for a fix, wipe your data, or wait for someone to tell us an embarrassingly obvious place to find a working option for this.