Switching to new BYOD and ESIM

I have 2 Network lines (one for me and one for my husband), we got the SIM yesterday but it is linked to our old iPhones and we now have new ones we want to use, they need an ESIM though and when I try to update my device via the app or online it gets through to payment (which is $0 as it is a BYOD) and it then just gives an error. If I try to generate an eSIM it goes to my old phone.