if i...

guys if i use the MVP train to unlock weapon mods, and later use them in online mode, will i get the cheater tag? or it is just for weapons?(dlc)

This belongs to the MVP thread since it's about MVP. Anyway, you will not get cheater tag unless you use any of the unowned DLCs (weapons, weapon mods, masks, etc.) or spawn in items etc.

Unlocking weapon mods in the base game or in DLCs you own will not get you marked.