[Help me please] I have trouble whenever i flash a new rom? topic

Hi im new to flashing roms and everything so i have rooted my phone and downloaded twrp and have flashed multiple roms to my phone but switched back to stock because whenever i flash a new rom i do not have a signal and it does not read my sim card i have tried backing up my stock then just recovering the modem but still no luck if any of you could help me it would be much appreciated I have a N7100 on under a Koodo contract.