Unfortunately this is all too common. Insurance companies just want to cut costs and if they see an opportunity to not pay they are all over like a.... Well you know.
You want to appeal and you want your doctor to make the argument (with support from the pump supplier). There is a process. This happens all the time and is a continuing problem many of us have here, and not just for pumps but all kinds of diabetes stuff. And it doesn't just get solved once, it is a recurring problem that you may have to deal with every year or new insurance carrier.
Just get your doctor involved and follow through with the process and you should be able to get this resolved. Your doctor just needs to write an appropriate appeal justifying the medical necessity. Often the pump supplier is very experienced in the insurance shenanigans and can help as well.
And take a deep breath and remind yourself you will surmount this obstacle and it will be ok.