I've performed "Syslog Port and Syslog search" within the MX2800 and MX2820 Manuals as well as access the ADTRAN Lab systems and I can find no mention/option to set the port 514.
I forwarded your inquiry about Syslog port 514 to the MX2800 Engineer and no reference to port 514 can be found.
The Engineer provided the following information regarding Syslog.
Configure the MX2800 for LAN operation (Valid IP, Gateway, Subnet). Have a Syslog server, such as the one in the ADTRAN Utilities software package, running on a PC. Configure the server to listen for messages on Local 0. In the Configuration>System Management>Syslog Setup menu of the MX2800, enter the Host IP Address of the PC running the Syslog server. Change the Severity level to Debug. Change the Host Facility to Local 0. Enable Transmission.
The MX2800 Product Engineer confirmed the UDP 514 is the hard coded Syslog for the MX2800.