[Asus RT-N11P A] Weaker receive WiFi signal than on stock firmware

The H/W label just says "A". I assume it's the same as "A1" (wiki page)?
I tested on stock firmware – definitely no such problem.
The device I connected to the router shows that the signal is strong, but pages only load at really close distance. When scanning for networks, this router only finds another router 1 meter away and says that the signal strength is 34%.
I tried changing various GUI settings – no luck.