This is the same question as here: posted by you.
How to create a reverse zone - Unsupported Modifications - Mail-in-a-Box Forum (mailinabox.email)
Did you manage to do it back then?
Never tried it on MIAB, as you are the owner of the IPs, but I believe the syntax in the zone is your IP in reverse order.
E.g. 192.0.2.255 would be stored under “255.2.0.192.in-addr.arpa”.
Maybe @MarthinL or @miabuser might help.
But this might be overwritten by an update.
You can bring your own IPs to CloudFlare if you own or control them and make a PTR record there. Read this: Reverse zones and PTR records · Cloudflare DNS docs
Note that in CloudFlare, If you are adding less than 200 PTR records, select the Free plan. If you are adding more, select a paid plan.
Add the two Cloudflare nameservers (or the MIAB nameservers) for the zone at your Regional Internet Registry (RIR).
What is ICANN Lookup saying about your public IPs. Are you the owner. Check there.