Hello MaRtYy01,
Okay, so, after re-doing the steps and flashing CM13, the 3rd build, the one you said it's stable and usable, put the IMEI back using MTK Droid Tools, the phone tells me something like "No SIM Card - Carrier". At one point, it connected for, I believe, couple seconds, I received a message, and after that, poof, nothing, it stayed the same. No SIM Card - Carrier.
Now, in this moment, my HTC Desire 310 has CM13 installed, it runs smooth, it works fine, Wireless works fine, but the SIM card is not working, with or without IMEI.
Kind regards,