I don't think moving the stock rubber-ducky antennas are going to do much for range or sensativity. But the reason they are on connectors is so that you can use your own antennas if you wish.
You can buy many different higher-gain WiFi antennas, all would come with the proper RP-TNC connectors on them (otherwise, the answer to your last question is an RP-TNC extension cable).
But if you are worried about range and sensitivity, I'd just go ahead and swap the rubber-duckys out with something external with a bit more gain anyway.
Cisco has a huge range they (re)sell...
But other brands/sources can be cheaper.
Just be careful of connector type, alot of the lower end consumer stuff switched to a smaller connector (RP-SMA), cisco stuck with the standard RP-TNC, although some bigger/outdoor antennas may have N, but cables that go from N to RP-TNC are readily available. (confused yet?)
2 meters incl. 2 additional connectors RP-TNC male/female will result in more than 2dB loss. This may be overcompensated by the better positioning of the antennas. The cable loss can be significantly higher. Rule of thumb: the thinner and more flexible the cable is, the higher the loss per length. Flexible low loss cable is quite expensive in small quantities.
I strongly recommends buying ready to use cables from professional dealers. You cannot use your soldering iron. The cable must fit physically into the connector for matching the impedance. Your signal uses 2.4 GHz not DC, forget about such things for 5GHz
I would place the 870 just on top of the rack as it is no 19" rack unit.
Remember that gain in antennas is always achieved by directivity.
An antenna with higher gain always has more directivity. When the purpose of the WiFi router is to provide wireless access inside a building, this may not be what you want. With a suitable pattern it could be usable for access from a single floor, but that is about it.
Antennas with gain are mainly suitable for point-to-point links.