it is from a reputable brand, and also supports 5ghz (your modem also supports 5ghz).
Most wireless signal function on 2.4ghz, however on that frequency it is probably clustered with other signals such as mobile phones, microwaves, etc. On 5ghz there are less noise, thus less susceptible to interference, delivering a more smooth experience.
I have no other suggestion than getting a cable to get optimun speed or a new wireless adaptor.
Which models in particular are the best I have no idea, I'd say you go to a shop and ask.
Good luck with that.
Formerly known as gamer4evere
MPGH GAMING NIGHT 08/03/2015 - Town of Salem