Investigating "emergency calls only" / 0 SNR

I replaced my USB / headphone jack board ("sub board") because of problems with the USB-C port.

Now, the USB-C port works well, but the phone struggles to get a mobile network connection. Indoors, both manual and automatic network selection fail (automatic: "emergency calls only", manual "Couldn't connect"). Outdoors, I get a 4G LTE connection, but bandwidth and jitter are poor, and I get 0dB SNR reported by the Network Survey app.

My guess is that there's a problem with the antenna cables.

Things I have tried so far:

• Reseating both ends of the coax antenna cables (white and black)
• Inspecting the cables, the PCBs, and the antenna cable connectors for damage
• Changing network mode to "NR/LTE/GSM/WCDMA"
• Reverting to the original sub board, to try to rule out a problem with the new board

Does anyone have any suggestions what else I could try?

This is the replacement part I used: https://www.ebay.com/itm/285156938467

And this is the guide I was following for disassembly and reassembly: https://www.youtube.com/watch?v=Ue6e6FOL...

One thing I noticed is that the replacement board is slightly different to the original – there's an additional antenna port on it (labelled "L8"), and there's seemingly a microcontroller missing vs the original. Is it possible that there are different hardware revisions here?