cheating? or overthinking

My husband refuses to let me see his phone. he's very secretive about it. whenever he's away from me (work/"mj"/dinner) he always don't reply me nor does he answer my calls. Even when I gave him an ultimatum to let me see his phone or I'm moving out with our daughter. he still refuse to let me see! I've my suspicion that he's cheating but I've no hard evidence!!! he changed his phone pw etc. What should I do? I thought of hiring PI but it's really expensive.... #advicepls #pleasehelp