The reason I didn't put a link in the previous posts was to get you to read about what you were doing as you searched for the modem. I am offering advice on things I have yet to experience on this device and only speaking second hand. My advice is only based on what I have read not on anything I have had to correct myself.
In truth I think you just stopped too soon in the process of what you were doing earlier. You said you installed the 4.3 bootloader in preparation to installing a 4.3 or later ROM that required the up to date bootloader. You didn't mention installing the current modem/baseband. You didn't mention trying to flash whatever ROM you had been considering.
You may be able to correct your device simply by installing the 4.3 modem.
HERE is the modem thread, it is in the General Forum for our device. Make sure you pick the correct one. I don't know if your device will work well on the MJB bootloader and modem with a 4.1.1 ROM, from something I seem to recall reading I suspect it will.
Take this in steps. Install the correct modem, the one that matches your bootloader. See if that corrects you sim, data, and IMEI problems. If it does correct the issues, the next step is to decide if you want to stay on the 4.1.1 ROM or change to whatever ROM you had been considering. If you decide to change, make backups first. If it doesn't fix the problem while on 4.1.1 you may want to flash the version of the official 4.3 ROM enewman17 has provided on the development thread.
Take it one step at a time. Read about what you are about to do until you understand it well, all before doing anything. Make backups sequentially as you make each corrective step.
HTH.
