It all worked fine, until I decided to buy a new phone, which for some reason doesn't like TP-Link rounter. Sometimes it doesn't even connect to it and can never upload at 25Mbps through one 30cm thick brick wall. So I searched for alternatives, and tried the old 54Mbps Huawei Echolife HG520b ADSL router, with the router part disabled of course.
The results were impressive. Even though I used the same external antenna with both routers, the Huawei router offers the best signal. My phone could upload with max speed, even through two 30cm brick walls.
I'm not sure what's the explanation, as both routers (Huawei and TP-Link) consume the same amount of power, had the same external antenna, same test location ... the only logical explanation would be that the older router simply has a better/stronger Wi-fi module, while on the TP-Link they thought they could get away with a weaker signal and compesate for it with the newer N standard.