...

Something doesn't seem to be working, I can't get my instance to fetch from it.

I think something's wrong.
This doesn't work:

• https://sh.itjust.works/u/admin@dbt-p2p.com
• https://dbt-p2p.com/u/InEnduringGrowStrong@sh.itjust.works
• https://lemmy-czja7-u9009.vm.elestio.app/c/agora@sh.itjust.works
• https://sh.itjust.works/c/start@dbt-ptp.com

This kinda works, but I assume it's not what you want:

• https://sh.itjust.works/u/admin@lemmy-czja7-u9009.vm.elestio.app
• https://sh.itjust.works/c/start@lemmy-czja7-u9009.vm.elestio.app

It seems your instance is configured as lemmy-czja7-u9009.vm.elestio.app but you pointed a CNAME for your domain to that.
I'd imagine it needs to be changed in the instance config.