Hello nismo-gtr
I just realized this post is quite old and you may have found an answer elsewhere, but ny two cents worth just in case.
One of the many advantages of the G300NH router using DD-WRT firmware is the ability to adjust the output power. These settings can be found under the wireless tab, then clicking the advanced setting checkbox. Not sure what the max is, but default seems to be 20dbm which translates to 100 mW. I am going to look in to this a bit further, but changing this to 23dbm will double the output power to almost 200 mW.
Also, supposedly anyway, the wirless N standard promises great range & speed, so if your wireless card supports it, try using that.
Good luck!
Rusty