How to connect two Orbi Routers in different buildings to extend Wifi coverage

> [...] Now I cannot figure out how to turn off DHCP and setup a static
> IP address for use on my existing RBR50 network. [...]

   Visit http://netgear.com/support , put in your model number (RBR10),
and look for Documentation.  Get the User Manual.  Look for "Use the
router as a WiFi access point".

   Terminology: A "static" address is configured on the device itself.
What you configure on a (DHCP server on a) router is a reserved dynamic
address, not a static address.  Either one should fix the address of a
device, but some implications are different, so it helps to know which
you mean.  Most likely, reserving an address on the main router (RBR50)
for the RBR10-as-WAP would cause the least trouble.

> [...] One more thing that makes this more challenging is I only have
> one ethernet cable avaible from my internet service provider. [...]

   That's typical.

> [...] I tried hooking it up to a little 5-port switch to split the
> signal between the house and the outbuilding but that did not work. The
> switch killed the internet signal.

   "killed" might be an exaggeration, but connecting multiple devices to
your ISP connection would tend to leave at most one of them working
properly.  You can add a switch on the LAN side of a (NAT) router;
generally not on its WAN/Internet side (that is, directly connected to
your ISP).