iPhone 12 Face ID - Qianli Apollo, i2c i6s, MC12

Hi folks - first time poster here but used iFixit guides a lot over the last few years as a hobbyist. I’ll try to provide as much info as possible on my question but if I’ve missed anything out please let me know!

I bought an iPhone 12 off eBay that was stuck in recovery mode. I discussed with the owner before buying and he said it had never been opened, he was not aware of any water damage, and it had just happened overnight.

I was pretty sure I could fix it so I jumped on it. Can confirm it had never been opened from the strength / appearance of the adhesive. Water damage indicators all pearly white. Sadly as this was my first attempt at an iPhone 12 repair I wasn’t aware quite how strong this was and broke the screen.

Identified the issue as the earpiece flex cable. Phone booted fine without it. Replaced the screen with an original Apple screen with no issues. Initially kept the earpiece flex from the donor screen and everything fine except FaceID.

However, no FaceID was bugging me so I took it apart the other day and fitted the old earpiece flex to the donor screen. First boot was to recovery mode but after a restart it now boots fine and everything working - except FaceID. I’m sure this is the original flex and I’m pretty sure I didn’t damage it during the transfer (I’d say 100% sure but that would make me look like a noob I’m sure - which I am still to a reasonable extent).

I updated the IOS and restored with DFU mode in 3u tools but still no FaceID.

In Settings I have an Important Display & Battery message on main screen (I also replaced the battery while I had it open - for some reason the 91% battery I took from the donor phone seems to die quicker than the 72% “Service” battery it came with, as an aside).

In FaceID & Passcode I have no warnings. When I try to set up FaceID, I get to “Get Started” but then it says “FaceID is not available. Try setting up FaceID again later.”

Does anyone know what this issue is from the description please? What further diagnostic steps can I take?

I have an i2c i6s with various boards, a Quanli Apollo Interstellar, and an i2c MC12… I’ve used the Qianli to fix battery serials in earlier iPhones when replacing, used the i6s once I to check the dot projector on this flex cable (all green and activated) and not yet taken the MC12 out of the box. I’m a YouTube repair video lover and bought these devices after seeing them in many repairs but I’m not 100% (or even 10%) on the full functionality of some of these devices, or which particular add on flexes I may need.

Guess it must be something on the flex that’s dodgy as it was causing the stuck recovery mode in the first place. Bounced it out of recovery in 3u tools before opening it but it just kept going back to recovery. I’m not sure if I’d be able to get FaceID active using the donor flex and a tag on cable using one of the machines I’ve mentioned and a tag on flex? I have dot matrix flexes for the i6s but don’t think this is what I need.

Any advice would be gratefully appreciated, cheers!