Wrong Charge for Return of Sagemcom Fast 5688W Gateway

I returned Sagemcom Fast 5688W Gateway internet tower in August via UPS since I was told by TMobile they don’t take these back at their stores. I got proof of delivery but was still charged $377 for TMobile’s claim of non return. I keep calling TMobile to get the charge removed. I have been trying to get it removed for about 2 weeks now and keep being told it’s under investigation. I am at my wits end since my bill is coming due and don’t want to be charged for something that is wrong. Has anyone experienced a non return when it was wrong and how did you resolve this. Thanks in advance.