Wyse 3040 Legacy Boot

 Hello

I bought an wyse 3040 thin client for cheap locally. I has 1.2.5 Bios installed. Secure boot is switched off, but still UEFI only. How can I turn on Legacy/CSM Boot? I want to install an older version of Linux/Windows. I know its less secure, but Im going to use it as my small linux box, as its very small and efficient. There is no "Advanced Boot Option" tab, which allowed to switch Legacy on, on older system. Should I downgrade the Bios or is there a secret methode to enable legacy boot on this lil box?