what is the lowest php configure works with the latest highend theme

I am on a shared server that is set to 7.4 php and I want to upgrade my highend to
the latest version. I was already using 4.0.3(?)version and my website went down when the hosting company updated the php on the server my site is on. The error was caused from the highend theme so they were able to get the site up but they downgraded my highend to a lower version 3.8.3 which worked but I’m no longer running my site well. How do I activate and upgrade to the latest highend version without losing the site again? I’m set to 512memory with 180seconds, whatever that means to me. And/or do I need to go to a vps? I’m currently on a share site (been for years, it’s a pro version with unlimited everything? whatever that means to me) I don’t understand why the site went down with critical error message to begin with. Please respond as I’ve been in this bind for 2 weeks and my hosting company keeps telling me to contact the theme vendor. thank you, Lisa