Make sure Options/Login "Enable listening ports" is checked, and restart Soulseek if you change the port. From another device on the LAN, point a web browser to
http://PC-IP:2100 (if it's a laptop, double check it's the current static IP). If your PC and Soulseek are correctly configured, you should get a message about "connection reset", which is good because it means that the browser has reached the port and Soulseek has rejected it.
But your PC may be allowing LAN connections while blocking others from internet with the Windows firewall or separate security/filtering programs. If they are all disabled for the tests, the router config should be revised.
On the other hand, if the router configuration is correct and the message says something else, like "the site didn't sent data" or "couldn't connect" after some time waiting, and all firewalls are down, then it seems there's something to fix in the PC. A restart may help.
Tip: if the other device is a phone or tablet and you try several ports, open a new tab in the browser for each URL. I tried this before replying and reusing the same tab was often confusing.