How do I connect 3 routers to a Pace 5286 ACX FXN Modem/Router combo?

You're double natting. A double nat happens when you cascade more than 1 router.
https://kb.netgear.com/30186/What-is-Double-NAT
Fixes? There’s 3 options but only use 1.
1. Put the modem/gateway in modem only/pass through mode. This disables its routing functions. Probably not the best option for your setup. 
2. Leave the modem/gateway in router mode but put the Netgears IP address into its DMZ. This forwards the data to the netgear. You can either leave the modem/gateway’s wireless enabled or disabled. Just make sure its using a different wifi channel to prevent interference. Won't work in your situation as you have 3x routers you need to connect and not just one. 
3. Put the Netgear into access point mode. This disables its routing functions. Wifi/ethernet still works but it does disable some features.
https://kb.netgear.com/26765/Which-features-are-disabled-on-my-NETGEAR-router-when-it-is-set-to-AP-Mode

probably the best option for your situation. 

While the above should get you working, you're using a mix/match of devices/equipment. If it was me with those speeds? I'd pickup a cheap renewed RBK43 or RBK753. Then you could go pace---->netgear router (in cabin)---->satellites (in outbuilding).  You'd hardwire the satellites to the netgear router. That'd make a mesh and you could roam between buildings without issue and have one device managing it. You'd still need to connect the orbi to the pace router but then you could follow the above guide.  

PS. Those R6080 are pretty low performing devices and old.