Returning 5G hub problem

[WCAT-16]

I was with THREE 5G broadband for 3 years. At the beginning I had poor reliability (constant dropouts) and following advice in this excellent Forum I tried several different 5G hubs, and got good results. Customer Service was not helpful so I just bought new THREE hubs myself on eBay.
Now I have ended contract with THREE and returned 5G hub.  But although same model MC801A, it seems I have returned a different IMEI number hub to the original one I got 3 years ago.
I have spent a lot of time trying to explain to chatbot etc......   
But I am still being told I will be charged for not returning hub. 
Can anyone help ?

[WCAT-16]

To add to the list, although I closed the account in February, Three sent me a bill for March. I spent ages online with a Customer Service agent who checked and said it was a mistake and would be sorted out.  Now they have just sent me a further bill for April and marked my account for collection.

[JonathanB]

Hi @WCAT-16,

I'm sorry to hear about these issues with the return and closure of the account. I've responded to your PM now to direct you to some more help.

Thanks,
Jonathan

[WCAT-16]

Thanks for the reply JonathanB
I've spent 3 hours this morning (really!) with chats, and then phone call.
The outcome is that THREE is inflexible; and that until I return the MC801A router they gave me three years ago, the account remains active and they bill me as usual. And since I have returned an MC801A router with different IMEI number they are billing me £150 for the original router. And they will try to return the MC801A I sent (they have it in the system), but not sure if possible.
It is all my own fault (not keeping the faulty router for 3 years to send back).
And the very understanding and helpful (human) customer service person will try and get the recent bills cancelled.