@advocatux some time ago this workaround didnt worked for me too.
But now my issue has gone, I really don't know why. Like @Stefano wrote I did reset my APN settings and gave the same settings one more time. Settings are the same, but mobile data switch is accessible now (it is not grayed out) and my mobile connection works like a charm.
Thank you all for your help and all tips 
@advocatux yep, Im talking about this bug I also did it as described (stable->devel->stable) but unfortunately no effect, it doesn't work for me on devel too so switching between channels deos nothing in my case
@Lakota @Stefano APN is fine, it worked for me once, 'mobile data' switch (to turn on/off mobile data) was inactive but I could switch between 2G/3G/LTE and I had mobile data connection, but after some phone restart (or I flashed UT once again on my N5) it didn't work anymore.
I will try to investigate more, but currently havo no more ideas
I also bought Nexus 5 32GB ver.D821 few weeks ago. It works great, I\m very happy using it, only problem is non-functional mobile data (I know there is a bug with mobile data) but except this all other things working great.
How to join UBPorts telegram group with UT device like BQ E4.5? I have Telegram app on it but link to UBPorts group is not functional with it.