Somehow this is currently my last idea, because the wifi HW is working with OOS but not with LOS...
Sorry, I'm lost in that case is as well. Only idea coming to my mind is that probably something went wrong somewhere during the entire process. Just to be sure let me repeat:
1. boot to twrp, check current slot (let's say you currently are in A), flash OOS, flash current twrp then reboot to recovery
2. back in twrp check current slot again, you should be in B now (or in A if before you were in B). Now again flash OOS; no need to flash current twrp again, instead reboot to recovery
3. you should now be in LOS recovery which allows you to do a proper factory reset / format data etc. (current twrp versions apparently cannot do this, at least it never worked on my OP6)
4. from LOS recovery follow instructions to flash current LOS 19.1. After that reboot to recovery again
5. back in LOS recovery flash some gapps solution (MindTheGapps is recommended; I flashed NikGapps but that apparently has some minor issues with LOS; you decide).
6. Now it's time to boot to system. You now should now do a basic LOS setup. Make sure you don't set any screen locking (pattern, pin whatever) yet; do that as the very last step.
7. After that check whether you have Wifi. >> if Wifi is still giving you trouble I'm out, no more ideas
Else you can continue like this:
8. In case you now are having proper Wifi and everything else you can think of,reboot to *bootloader* then start *fastboot* on your PC and boot into TWRP (in case you want TWRP instead of LOS recovery):
Once you're in TWRP recovery you can set that up. Go to "Advanced" and choose "flash current twrp", then reboot to recovery. That should be TWRP now, and you should have unencrypted access to data
9. reboot to system and continue setting up
Good luck once again