Combine default gateway and RNRP on AC800M
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You can use both simultaneously.
RNRP setup routing to adjacent RNRP areas.
Since office network is not running RNRP you must define a Default Gateway in the controller so it know where to send telegrams destined outside what the routes RNRP has created in the routing table declares.
The router node between the client/server and office network is often what the controller should have as default gateway, but you could point it to the connectivity server and there use a new default gateway address to take you further.
Likewise, the time source must know how return a telegram back to the controller having a private and non-routable address like 172.16.80.151. This can be done with default gateway (pointing to the same router node the controller has as default gateway, or via a static route with a more narrow setting declaring that telegrams destined to 172.16.80.0/24 should be sent to the router between client/server and office networks.
I guess you could use a NAT rule or service to assist the controller's time request finding its way out from the client/server network and back (just as your router at home allow insiders to reach Internet from a private non-routable IP-address). NAT would be more secure as it blocks outsiders reaching the inside networks - why not use a proper firewall as router? It can do NAT and in the same also restrict traffic to certain protocols like SNTP only.