How to lock network operator or to never go on roaming, or get out of roaming as soon as possible I work really close to a country border, and I do NOT want my phone to connect to other operators than mine. 

Sometimes the operator from my home country will be unavailable for a brief period of time, and the phone will go on roaming. (and it's useless. no phone, no sms, no web. pointless)

When I'm back in a zone where my home country operator has coverage, the phone will stick to roaming for a good while (hours), and I have to notice it and manually select my operator.

Unfortunately when the phone reboots the "choose automatically" option is re-set, so if I reboot the phone (or when it reboots itself), I have to do this again.

I have tasker, but didn't find a way. I am also ready to root the phone if needed.
I just don't know how to select a network operator and prevent the phone to switch to "choose automatically" at every reboot. It's not about apn and roaming charges, I just want it to never go on roaming ever ever ever.

Any suggestion?

(device info: OnePlus One, android 5.0.1, cyanogenmod cm12)
It seems that Android M Marshmallow does fix this bug.

I am now running Android 6.0.1, CyanogenMod version 13, and the operator does not revert back to automatic-selection anymore after reset.

The exact CyanogemMod version I am running is: 13.0-ZNH2K-20160430-STABLE-Sultan-bacon.