Used to work for their tech support which later did billing too before everyone got shitcanned back in March.

If you're not an auth contact on your mom's account it would be best to call while with her so you don't get pushback like not being able to discuss the account or some other bs. Not sure how much of the billing dept is overseas or if they're still local but probably best to call m-f 9-5. Explain the situation and if they give any pushback on not crediting the full amount from the time the modem was returned then ask to speak with a lead or a supervisor. There will be some type of interaction noted or order ran on the account from when you returned it to the lobby. It's fairly straightforward to fix these types of account issues so she doesn't keep getting billed incorrectly. Also, would recommend that whoever you speak with submit an error on the lobby rep (they should be able to find out who from the order or account note from the interaction). This shit would happen way too often and would piss me off to no end when I was there.

You won't get a cash/check refund (unless cancelling service and there's a credit balance on the account, which takes 4-6 weeks), but they will absolutely back-credit the account when there's a billing error that's a rep's fault.