i try to understand your question but i think you dont actually know what to ask? so i think you asking is you have router (connected on internet "main WAN -Internet) and you have secondary WAN which is your 4g Modem, and you have 2 internet connections and if wan1 fails you want wan2 to work so...
try to consider this time that all people are at home using their mobiles, gadgets, etc. to connect internet you're not alone with that kind of speed, if that is via wired possibly there is no problem because it has a dedicated speed unlike to other uses sim cards. hope you find a solution for...