Okay, well if somewhere under domain management you cannot find where to create a glue record and where to change the nameserver record, then you may need to transfer to a different registrar.
The reason MiaB is configured this way is that the other DNS records that MiaB creates are higher in number and easier to make a mistake on, so having MiaB create the DNS records is the easiest option.
Even a reseller should be able to support these changes to the domain records. It’s possible they can resolve this issues through a support ticket, but yes perhaps they are not open today.
In your post above you were on the correct page for changing the nameserver record, but you still need to create the glue records so those URLs are pointing to something, and as we just discussed this cannot be performed from the DNS server - it has to be somewhere in the domain management area.