I think I am starting to understand what you are saying, but I'm not completely sure. See if this is close to what your situation is ...
For example purposes, say that your ISP has the ISP.COM domain allocated to it in DNS, and has created the subdomain MABDRASOL.ISP.COM for your use in which you can define your own DNS resource records. Obviously, the DNS records that you create must be either literally named mabdrasol.isp.com or have a prefix such as abc.mabdrasol.isp.com, def.ghi.mabdrasol.isp.com, etc.
So, continuing with the example, say within your MABDRASOL.ISP.COM subdomain you originally had 3 A records with DNS hostnames defined to resolve to IP addresses as follows:
www.mabdrasol.isp.com A 1.2.3.4
ftp.mabdrasol.isp.com A 5.6.7.8
smtp.mabdrasol.isp.com A 9.10.11.12
Then, later you decided to migrate those A records so that they could resolve on your GTM instead of your ISP. So, the approach that you took was to first define them on your GTM with each being a separate zone in ZoneRunner:
www.mabdrasol.isp.com - defined as a zone, with an A record of 1.2.3.4
ftp.mabdrasol.isp.com - defined as a zone, with an A record of 5.6.7.8
smtp.mabdrasol.isp.com - defined as a zone, with an A record of 9.10.11.12
Then, at the ISP in your MABDRASOL.ISP.COM zone, you replaced the A records with NS records to achieve delegations as follows:
www.mabdrasol.isp.com NS gtm.mabdrasol.net
ftp.mabdrasol.isp.com NS gtm.mabdrasol.net
smtp.mabdrasol.isp.com NS gtm.mabdrasol.net
where, again for example purposes, gtm.mabdrasol.net is the DNS hostname of your GTM.
Everything now works fine with that arrangement. When you want to make a change in how the three records resolve (e.g., perhaps have abc.mabdrasol.isp.com resolve to 4.3.2.1 instead of 1.2.3.4) you can just make the change directly on your GTM without having to utilize your ISP.
But ... now say that you also have an MX record at the ISP:
mabdrasol.isp.com MX 10 smtp.mabdrasol.isp.com
and now you find that attempting to use same approach that you did with the above A records fails.
The reason it fails is because to implement the needed NS delegation record:
mabdrasol.isp.com NS gtm.mabdrasol.net
you would have to create this record in the upper-level ISP.COM zone itself, which is under the exclusive control of your ISP. You cannot create any DNS records there.
In short, this is because mabdrasol.isp.com itself is a DNS domain name, whereas www.mabdrasol.isp.com, ftp.mabdrasol.isp.com, are DNS host names.
I hope that this addresses your situation ... and that it helps.