It can take up to 28 days for the return to show. In addition to this, there have been postal issues in some areas due to reduced staffing at sorting offices due to the current covid situation. For example, here in liverpool we have seen a 2 week delay in some areas for post to arrive.
Once received they will be logged in the system. The devices are linked to your old account via serial number.
Please log in to My Account if you need to view or pay your bill, manage boosts and track your usage. From My Account you can also check your connection and test your line for any issues in the Service Centre.