Hi,
These one of these groups might be more appropriate to find an answer:
/search?q=D-Star+gateway&p=SubsCount%2C%2CD-Star%2C20%2C2%2C0%2C0
I'm sure you know it's not as simple as your post is showing otherwise you would not be asking.
1)? Satellite link, VIASAT, Hughes Net, INMARSAT, Iridium or maybe Starlink, all expensive and still Internet.
2)? Lease line, probably the best option as a RF solution can cause Intermod interference? Still expensive.
3)? Use a band that your repeaters do not operate on..
4). WiFi or other line of sight method.
5)? Yes on 1.2 GHz but not data modules.? You can not add a 1.2 GHz data module to a D-Star repeater.? You'll need a 1.2 GHz? transceiver.
Sorry I do not have a specific answer.? Hope? the above helps.