Is the phone GSM or CDMA? If it's CDMA I don't think it can be done. I'm in that position with an iPhone SE bought through VM which uses Sprint. Phone can't be used with straight Sprint, just VM. I believe xfinity was working on unlocking the CDMA phones so they could be used with xfinity mobile, but I haven't read anything lately on that.
We are having this exact same issue! It has been very frustrating and nobody at xfinity or sprint can give us answers. I'm hoping they will figure it out soon. Have you figured anything out yet? If they are advertising that sprint iPhone se's can work on their network, then they should actually work.