Default [Q] replacing modem.img within custom ROM (zip) --> (both) SIMs offline

After lots of reading XDA and using the many tweaks and fixes one can find here (thanks!) I ran into a problem I can't seem to solve.

I'm modifying a custom ROM (by www.jiayu.es --> version 20131203-000001) for the Jiayu G4B (basic 1/4 3000mah) for use in the Netherlands. I already applied some fixes (GPS-fix, APN-settings, SMS-fix, add some APKs) and created a flashable zip file for use with CWM (thanks to dsixda kitchen).
One thing I can't seem to implement is the replacement of modem.img (in system/etc/firmware) with an updated version that fixes frequent drops of the connection to the network. This fix is documented on the dutch site tweakers.net.

The original fix is quite simple: you just replace modem.img (using a file explorer) with the updated one, straighten out the file permissions and reboot the phone. The thing is I don't want to have to do this manually everytime I flash a new ROM, and furthermore it's fun to learn how to customize my own ROM.
So I thought I could just replace modem.img in my working folder with the newer one and be done with it. Problem is that after I flash the ROM containing the new modem.img both my sims remain offline and the only way to go online again is by restoring a CWM backup.
I already checked the file permissions and they are correct. Unfortunately google also couldn't help me find the answer. Does any here have an idea how to solve this?