That's pretty look like your cabinet is bit far away from G.fast. You are better off stay as FTTC with Zen Internet but have to warn you if your G.fast has completed migration to Zen from TalkTalk then u are stuck with 12 months contract as Zen won't allow you to downgrade to FTTC until your contract has completed first.
I think OR Engineer are correct as it look appear you are around 300m or more away from the pcp cabinet to your property for G.fast (don't get mix up with Fibre Cabinet and PCP Cabinet)
PN FTTC 80/20 since 2014