RCM for composition dealer on rent

Hi everyone, I have a client who is running a hotel business and into Composition scheme. From July RCM may come into play. So client has to pay 18% tax on rent and pay to GST? If yes how does it work? Same like general tax payers, where we pay tax first month and get refund on second month and utilize the same for second month. Pay tax for third month and utilize the same for fourt month? really need help on this issue.