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.
I just put my Xfiinity Mobile SIM in my Unlocked Verizon iPhone SE and it can connect to 1x 3G LTE, but can't get data nor calls, and calling out to any number leads directly to the Xfinity Mobile Activations line.
Interesting, because my iPhone 8 from T-Mobile isn't "compatible" due to missing CDMA radios which the iPhone SE has and yet the iPhone 8 works with everything (sans CDMA). Xfinity Mobile's official statement is "iPhone 6 and newer" which the iPhone SE is two years newer, but apparently unsupported?
Is your iPhone SE still working on Xfinity Mobile and if so, what software version does it have?