To start Remote Play while your PS5 console is in rest mode, select Settings > System > Power Saving > Features Available in Rest Mode. Then, turn on Stay Connected to the Internet and Enable Turning on PS5 from Network.
How to start Remote Play from your PS Portal
On the home screen of your PS Portal, tap the portal or press the cross button. Your PS Portal connects with the PS5 console you used previously, and Remote Play starts.
Your PS Portal won't be able to search and connect to your PS5 console if it's been a while since you last signed in. Before trying to connect remotely from outside your home, check that your PS Portal connects properly while you're near your PS5 console.