Please read the manual as everything is well explained: http://www.tp-link.com/Resources/document/TL-WR841N_ND_V8_QIG_7106503891.pdf

I am not sure changing your antenna would change anything in your case. Have you considered using a Power Line adapter ?

TL-WR1043ND V1 firmware was recently updated 2 days ago (April 28th) with the following bug fix: Improved the login policy to enhanced security; Improved media server function to support 3000 files;...

Can you check if your antenna are properly attached ? Finally what type of WIFI card is used on your computer ?

I wouldn't set a static channel to 5 as this prevent the WIFI router from selecting the best channel with the least radio interference. Moreover, try narrowing your bandwidth from 40Mhz to 20 Mhz,...

I have raised the point to the TP-LINK support as a product improvement, as checking for the best WIFI channel (with the less interference) could be scheduled on a daily basis... I will let you know...