how to make a window 10 recovery disk + W10 license [duplicate]

I need to downgrade from 1903 to 1803. Since I updated over 2 months ago the only option I have is a brand new install from an iso file. The question is, if I use this iso file can I put in my OEM Windows product key or will it not work? I cannot use the fresh install because it would still have 1903 and this causes my system to crash.

If Windows 10 came preinstalled on my machine, can I reinstall using a Windows DVD?

A Windows 10 allows you to install any version of that edition on your machine. Since there is no concept of an "OEM" Windows 10 ISO, you can use a retail Windows 10 ISO, to install an OEM installation. When prompted for the product key all you have to do to install Windows is skip entering the product key.

The question is if I use this iso file can I put in my OEM Windows product key or will it not work?

It absolutely will work.

I cannot use the fresh install because it would still have 1903 and this causes my system to crash.

You can simply download a Windows 10 version 1803 ISO directly from Microsoft. I won't suggest which method you use to download the ISO, but all the linked solutions, download the ISO directly from Microsoft.

The only times you will be limited to Windows 10 version 1903 is if you use the Media Creation Tool or the Upgrade Assistant. If you find yourself installing Windows 10 more than once a year, you should keep a version of each tool for the version you want to download, available to be used. Existing copies of the tool will download whatever version of Windows 10 they were released for.

Worth pointing out that downgrading to 1803 will require you to perform a clean Windows 10 install. Windows 10 only supports upgrading to a feature update that is newer. There is a limit on the number of feature upgrades you can skip, I had problems trying to upgrade 1507 and 1511 to a newer upgrade (it would fail during the process), I was forced to upgrade to 1607 before newer upgrades would work.

Windows 10 Professional version 1803 will reach the end of Service on November 12, 2019. I suggest you upgrade to Windows 10 version 1809 instead which will give you until May 2020 to figure a solution to your Windows 10 version 1903 compatibility issues.