How to handle DNS for local secondary gateway servers for customers without any DNS services.
One solution could be to use a DNS service of your choice (ex: https://dyn.com/dns/ ) you can create your own 'Partner DNS service' and then create DNS records on behalf of your customers, for those that do not have their own DNS server.
Say for example, you have customer "Acme" and they have a secondary server locally on 192.168.0.10
Create a new A record in your DNS service like:
server1.acme.partner.com A 192.168.0.10
Then a users PC workstation, or an MFP/printer (if configured properly for DNS), would be able to resolve "server1.acme.partner.com" to IP address 192.168.0.10