GOOD DAY

Please help.

I have a raduis server running on my 1 Mikrotik.

The I have userman running on a second mikrotik.

it is on 2 different networks.

my isp is unable to suplly me with static public IP.

but I was able to setup ddns.

How can I configure the second router with userman to use the remote raduis server?

If I can get you clearly, you have a radius server (userman) running on router one and want to use the second router as a radius client connecting to the 1st router?

If that is what you mean, then you need a public IP on router with the radius server (userman), since the both routers are not on the same network.
Alternatively you can install CHR on Amazon AWS or Linode server and use it as a radius server, since they will give you a public IP.