No 4g connection

For several weeks now I've been having connectivity issues with 4g. It just started constantly dropping 4g or when in 4g it has no connectivity. It seems to work fine enough in 3g but the 4g coverage was one of the selling points for keeping with verizon on my most recent contract. I have replaced the phone twice now along with SIM card but there has been no change. I dont know what else to do.