When all three Towers are activated in the maps Tranzit, Die Rise and Buried, you have to connect those locations by inserting the NAVcards. The player can only hold one Navcard at a time, picking up a different Navcard while holding another Navcard will replace the existing Navcard the player is currently holding. So make sure you insert the right NAVcard in the reader, before continuing collecting other NAVcards.
The NAVcard in Tranzit can be found in front of a dumpster behind the lavapool on the right side of the Busdepot. The NAVcard in Die Rise can be found in the same room as the PDW-57 wallbuy, near some short cupboards. In Buried, the NAVcard is located on one of the bookshelfs in the Mansion, upon coming from the Maze side.
Take care you do this under the same online player account. When you have found a NAVcard, you have to return to the NAVcard readers in another map to insert it. The Tranzit card can be inserted in the Die Rise reader, the Die Rise card in the Buried reader and the Buried card in the Tranzit reader.