A gente ficou perdidos na mata, sem sinal no celular, não podíamos ligar para ninguém.
-We got lost in the woods/the forest with no mobile phone signal.
-We got lost in the forest with no cell phone signal.
-We got lost in the forest/in the woods, where tall trees totally obstructed any cell phone signal.
(and other ways, to say that, these were just some of them.)
We got lost in the forest with poor (or weak) signal. (here, it means that the call might drop out, the voice quality is so poor that you can´t understand it sometimes etc...)
We got lost in the forest at a location in wich the reception (signal) was so poor that we couldn´t make out what people on the other end said.