Enable High Power Mode and set the highest power and see if you can get proportionate signal strength. If you get no luck, RAM as #2

nop , you can't. WA5210G is an access point and it has only one wire port. you need another router or adsl router deployd as below internet-----[wan]router[lan]-----WA5210G ))))))

Antenna is a passive device so itself doesn't generate any power. What antennas do is radiating the power (feed from the AP) with specific pattern. Well, roughly speaking, we can imagine radio...

As for you condition, you won't need any external antennas if you buy the TL-WA5210G because 5120G has 12dbi built-in directional antenna !! But note that it's directional outdoor AP, you will need...

It will be 20km between every two stops. However, it's still too long. Even if we use TL-WA7210N with TL-ANT2424B, I think 10 stops is needed and we need professionals for the installation. Every two...

It's all depends on the DD-WRT team. I am also hoping they can develop firmware for more models~~