In late december, I returned our issued router that we have been renting since may of 2017, to our nearest FIOS store. It still has not been reflected on the billing information as of Feb 1st.
This exact thing happened to me. I had to call customer service several times about the same issue. I had to find the recipt/slip they gave me when I dropped the router off and I was asked to go to both the store I dropped off the router and the main office in my area all to stop being charged for the router I returned. I took 3 months to stop being overcharged and then I finally recieved a credit on month 4 of the original over charging issue. I strongly believe this proceess is delibratly hard to correct in hopes you give up.
I need to keep a close eye on my verizon account due to many unjustified over charges. Usually I call customer service were I have to argue with a customer service representative for 20-40 minutes to take off the over charges.