AAA RADIUS UDP Port Number

Modern RADIUS implementations use UDP port 1812 for authentication and authorization, and UDP 1813 for accounting. Older implementations used 1645 and 1646.

Thus, when configuring a Cisco IOS device to use a RADIUS server, the appropriate ports must be specified, like so:

SW1(config)#radius-server host 192.168.1.101 auth-port 1812 acct-port 1813 key radiuspass

The ports specified in this command should be the same as the ones used on the RADIUS server. Most RADIUS servers should use these ports by default but you always have the option to change them.

https://networklessons.com/cisco/ccie-enterprise-infrastructure/aaa-configuration-cisco-switch/