Open WRT on QNAP Virtualization Station

I have a Qnap TVS671 with i5 CPU, 16Gb memory and 6x NIC.
I have been using Sophos UTM running in Virtualization Station fine for 4+ years. but it is not supported anymore.
I would like to install Open WRT , but i need a ISO file/image for installing, it does not support IMG images.

Is there a way to convert a OpenWRT img to a ISO file? i tried some googeling and suggestion but i cannot get it to work.

You can simply change the extension to ISO. They're identical file formats.

I thought also it was only extension change, but I get error: Boot Failed: Could not read from CDROM (code 0004). No Bootable device

I use the following image:

I run now on a Raspberry Pi 2B
OpenWrt 23.05.0 r23497-6637af95aa / LuCI openwrt-23.05 branch git-23.236.53405-fc638c8
But the troughput is "only" 30MB up/down, where I have 40/200 cable connection