It seems your app still doesn’t have a shared IPv4. Here’s an example from an app of mine
[I] ➜ fly ips list -a rinha-nginx
VERSION IP TYPE REGION CREATED AT
v6 2a09:8280:1::2b:c07d:0 public (dedicated) global Feb 23 2024 15:59
v4 66.241.124.106 public (shared) Jan 1 0001 00:00
You might want to try fly ips allocate-v4 --shared and use the A record on your certificate because unfortunately the internet is not ready to work just on IPv6.