tl;dr : The RT-N15U does NOT have a u.fl connector and an external antennae mod, if at all possible, will be challenging.
This was supposed to be an easy mod judging from whatever information was already out there on the internet. Pop off the cover, attach u.fl pigtails to U523 and U524, screw on the antennae and mount the board.
Unfortunately, things went wrong from the very start. The board was screwed on to both the front and back of the case and the screws were not accessible from the external part of the case. I had to break a portion of the plastic off the case, subjecting the board to an unnatural degree of flex in the process. The broken piece of plastic can be seen in the bottom left corner. Thankfully, the board sustained no damage in the process.
Now that we have a better look at the board, we have ceramic heat sinks covering what I guess are the CPU and wifi chips. The uncovered chip on the left is an MXIC serial flash chip and the one at the top is a RAM chip. To the right of the board, there are connection points for serial TTL marked by J2. We would have to solder on male connectors to make any use of it.
Getting down to business, U523 and U524, located on the left side of the board is rumored to be a u.fl female connector. However, no amount of forcing could get my u.fl connectors to fit. Upon much closer examination, these aren't u.fl connectors. Yes, internet can be wrong sometimes. Further research into similar embedded devices revealed that it is actually a proprietary connector. It is most likely the MM8430-2610, and is used for testing purposes. At 50 bucks a pop for the external probe, I am not going to take that risk.
In comparison to the Aztech DSL1015EN, I would say that this board is much more unfriendly to tinkerers. The Aztech was easy to unscrew, had serial TTL connectors already soldered on and used industry standard u.fl connectors.