I don't know what version of radius server you have installed, but have you set up a client for this hotspot in the clients.conf file? I use freeRADIUS, and there are examples of how to set up a client in that file.
FreeRADIUS has a test program called radtest. I use it to test the setup on the RADIUS server. I must be root to use it.
radtest user password xxx.xxx.xxx.xxx 0 radiussecret
Replace user and password with valid user/password of course, and xxx.xxx.xxx.xxx with the IP of the RouterOS hotspot box and radiussecret with your radius secret. If all is well it will respond with what it will send your RouterOS box.
If the hotspot and the radius server are in the same box, use 127.0.0.1 as the IP.