I had the same problems with built-in node. Never got more than 5-6 connections. Then I decided that is better to go on VPS rather than running the node locally. There I have 17-18 connections all the time, and I don't need to keep my PC on 24/7.
If you still want to run the node locally, you should have a look in here: https://idena.io/?view=guide#guide-issues-1