Using User Manager to authenticate via Winbox

So here’s what I’m trying to do:
I’ve got two routers; let’s call them AuthenticationRouter and OtherRouter. I’ve set up User Manager (plus accounts and routers therein) on AuthenticationRouter. I want OtherRouter to look to AuthenticationRouter via Radius to authenticate Winbox logins.
Radius is sending requests from OtherRouter to AuthenticationRouter, but they’re coming up with error “received remote request from [OtherRouter’s IP] with unknown address, dropping”.

Any ideas what I’m doing wrong? Is this even something that User Manager is capable of doing or am I completely off base?
Thanks in advance for any help.