No there's no way to keep it running 24/7 besides buying the biggest plan. Don't worry tho your friends can join the lobby server and just type "/join yourservername" at any time. If it's online they will get connected immediately if not it should start in a couple minutes and they get connected to it once it's fully loaded. If they somehow don't get connected to it just tell them to reuse /join after like two minutes. Sometimes if the server takes longer to load than usual Minehuts system won't auto connect them anymore.