Hello Aternos!
You guys have quite an amazing thing going on, with the ability to allow players to have free minecraft servers. I know obviously it isn't easy to pay for such a thing, so I have a few suggestions that you guys can take if willing to be able to help with server upkeep, securing more server space for the servers to serve those who get servers for you, and the like.
1: Add a Donate option to the website, giving generous players the opportunity to donate to you directly.
2: Make a Patreon page, so people could if willing donate to you on a monthly basis, giving you an assured basic monthly income for your server. As rewards for their pledges you could offer things like free keys every month, or a weekly or monthly chat with one of the employees here.
Patron Link: https://www.patreon.com
3: This one probably won't happen, but I will list it here anyway. An option for a fee paid, to be able to keep your server online when no one is on for a time. This allows people a bit more flexibility, like if they are coming back in an hour and don't want to wait in a queue on Saturday morning, you could charge a small fee to allow them to keep the server up for the time being.
4: Try adding to some places on the website in empty spaces some ads, to boost ad revenue. As long as it doesn't cover the control board, players would not mind such a thing.
Anyway, Thanks for reading these suggestions!
B35Patriot