So it sounds like you are still PXE booting with the legacy network stack. UEFI has a different network stack that you need to enable and boot from. Depending on the vendor, the setting is usually something like PXE Internal IPV4/IPV6 or IPV4NetworkStack. When you boot with this, you are booting the system in UEFI mode and ConfigMgr will reflect that by setting _SMSTSBootUEFI=True.