The Geo-Filter uses a database to map IP addresses to locations. Unfortunately, the database can sometimes have the incorrect location for an IP. The ping assist is a feature introduced in version in 1.02.7 designed to address this problem.
The Ping Assist slider is set in milliseconds. The router will look for any hosts that fall below that value and allow them regardless of distance. For example I live in the UK & set my ping assist to 40ms. I sometimes find games on the West Coast of the USA which is way out of my Geo-Filter but the ping is below 40ms. We know for certain that this is an incorrectly located server but we didn't miss out on the chance to get a great game because of this.
Continue to set your distance as you did before but now set the Ping-Assist slider to what you feel. We recommend setting it above at least 30ms or it probably won't find anything unless you have a really low base ping. We personally recommend between 30-50ms.