I'm leaving here my way to get my signal back after flashing Resurrection Remix
I fondled 4 evenings with this fu***** thing and finally i found my solution.
My problem was the following: after flashing the rom, my phone was havind some seconds singnal, then minutes no signal and for about 2 seconds signal again. Different Radios and everything, nothing seemed to be the right solution. On stock rom everything was fine again, so i decided to look at the efs a little bit closer.
My IMEI always was the same as under my battery. but my Serial number was different as the one from the stock (it still is, but whatever, still working) and i never had my IMEI or serial number filled with 000000.
After going back to the newest stock rom and macking a full backup with efs (TWRP) i thought i could restore the efs after flashing Resurrection Remix. Good thought but still no signal. We have to go deeper. I was looking over all files in my efs and in my backup.
I wanted to copy my backup manually into the /efs, but couldn't write into the directory. So i did "chmod 777 *" in the /efs and *zapp* my network directly started to connect and was staying connected.
To make everything a little bit more oranized i flashed Resurrection Remix again, restored efs and then did directly "chmod 777 *" in the /efs and it worked like a charm.
I saw someone had the same problem but no answer. I can't find the thread anymoe, so i hope this is maybe helping someone in the future