Hello,
I had the contract where you got an EE mobile broadband backup. It and my router were plugged in. The openreach engineer, after installing my full fibre, handed me the old equipment but it seems to be missing one of the ac adapters for either the router or the EE broadband thingy. I've checked my setup and there isn't an ac adapter left behind that isn't connected to anything. What will this cost me?
Just send back all the "bits" that you do have. I can not be certain but I doubt that BT will be that bothered that the power adaptor is missing.
Even where devices are refurbished, it's pretty much industry standard to replace the power adapters, as replacement is often cheaper than testing! So as @gg30340 says, I wouldn't worry.
What I would do is take a photo of what you send back and keep the receipt!