router - how make this two 'separated' lans/networks see each other/works as one?

Question

https://i.imgur.com/VUEkRbO.png

i need that PC1 and PC2 see the others PC's ans if need access the shared folders, there is a way to do that with is modem 'locked' and with no possibility to change his configs?

Answer 1

等待大神解答