Greetings, all!

Just wanted to post this in case it might help someone in the future. I had a call for a 2016 328i to add a spare FOB, and I had a few renewed HUF5676 keys that I had renewed with VVDI Key Tool sitting in my van. On site, the FEM unlock went fine, the transponder programmed, but the FOBs (three of them) had no remote/comfort function. They all had good batteries, were sending the right signal, etc. I ended up flashing the FOBs again but this time with HexTag, then running the unlock process again, and I had no problem. Not sure if it was the unlock that was the problem or the FOBs (someone on another site mentioned that vehicles with updated ISTEP will not take the VVDI flashed keys), but AHII/HT got it done.

The proces of unlock again FEM was useless...once you have the SECRET FILE generated by autohex...you must be sure that the key was the problem not the secret file...so...100% key problem

Maybe I missed something but never experienced even single success after renewing second hand keys 434mhz for FEM. I tested plenty of them. I'm using Xhorse or KD or Hextag. Always the same scenario as you described. Car starts on emergency but keyless and remote not working. Changing keys positions or using different machines never help as well. Keys 868 however always perfect. My solutions are China dedicated keys.

I've tried once with unlocked HUF5676 with hextag for FEM on a 2018 M3 and it worked fine without any issues but maybe I got lucky

The proces of unlock again FEM was useless...once you have the SECRET FILE generated by autohex...you must be sure that the key was the problem not the secret file...so...100% key problem

I have no reason to doubt your expertise, but I flashed reflashed the FOBs before going back to the job, tried programming them (again, the transponder worked but the remotes did not), and then only had success once I re-unlocked the FEM. What you're saying makes perfect sense, I am just saying what I experienced.

I also had the problem when I test a locked 434 MHZ key.

I have renewed him, and he is displayed as unlocked but the learning fails.
I must say that I am an absolute beginner.
And the whole thing was whit testplatform on bench.

Can I change the frequency in the FEM and use a 868 MHZ key ?

you can change the frequency in fem, the key will only work on the coil, it will not work by radio. the frequency change in fem bdc is only for you to be able to program the key.
fem then you change to the frequency that the key has, if you want to have a working remote control, you must have a key with a frequency of fem bdc