ISPs should be warning people that if they change ISP, they could loose their email account
But you haven't. What you've done is blindly rushed forward without following information supplied, even on this topic.
I've had a Gmail account since the days of the trials, where a friend had to invite you to join. I've changed ISP's several times but never lost access.
Read what people are telling you here, especially the one here
https://forums.thinkbroadband.com/general/t/4635939-... (click the link to read it) - gary333 has given the steps needed to get your account back.
It has nothing to do with changing ISP, the ASA or reading any sort of guide. It's because you have ignored warning messages.
Can't make it any clearer than that.
EE Fibre Plus