poor wifi since changing to KPN

Hi all,

I just moved from a flat with Ziggo as the internet provider to one with KPN. The provided Experia box from KPN gives a shitty wifi signal in the back bedroom of the apartment; I tried hooking up my TP link router (was like 80 euros, so not a shit one) to the Experia box to try to improve the signal, but it's still impossible to stream videos in the back bedroom. When I called KPN they said I could buy their Wifi repeater for 100 euros, but that it would make the whole network slower. Has anyone had these issues and knows a better option to improve the wifi signal from my back bedroom?