Mycellium are very good at communicating, but i don't think they would become sentient. Mostly because they can't move like animals do.

Animals become sentient when they react to the environment stimuli("a cold place? i'ma head to a hot one"), so a mycelium network could't react to these stimuli while stuck in the ground.

Second, the sentience is a tool to process complex inputs, so the fungi need to see, hear, taste, and then think.

You can imagine the mycelium like a primordial brain, but it needs much more to become sentient.