Help on how to boot from old SSD on newest Motherboard

I have a PC with a motherboard ASROCK H510M-HDV/M2 retail and a disk SSD Patriot P300 512GB NVME M.2 PCIE GEN 3 X4.
On hand I have an older SSD Apacer 240GB with Windows 10 Pro on it.
I want (and need) to set the old Apacer as the boot drive but :

In the BIOS
1. It is recognized as a SATA disk
2. It does not show up in the boot menu. Only the newer NvMe is shown in the boot manager.
3. When I remove the NVME disk, there is no boot disk available.

There must be some BIOS tweak to make the "legacy" SSD as boot disk but I cannot find it.
Any help appreciated.