Is there an outdoor antenna suitable for my NETGEAR NIGHTHAWK MR2100.

You have posted your message in the section of this community given over to Nighthawk WiFi Routers. (This is easily done, given Netgear's impenetrable community structure.)

You might get more help, and find earlier questions and answers about your hardware, in the appropriate section for your device. That's probably here:

Mobile Routers, Hotspots & Modems

I will ask the Netgear moderator to move your message.