What is the purpose of Port 853?….

DNS. The protocol or service generally associated with this is DNS over TLS…..

Transport Protocol

TCP

Safety precautions

Control which solvers are allowed in the company….

Should we open this port in the firewall?

Only if a service is actually needed the use it. Apply the principle of any privilege: limit the authorized sources, destinations and interfaces, and then log connections when relevant.

How do I check this port?

Test-NetConnection SERVER -Port 853

Under Linux, you can use nc -vz SERVER 853.

Port/service associations are common conventions. An application can be configured on another port.