Antivirus vs My VB.Net app

Job ID: 36767274

Budget: $10 – $30 USD

I have a small program, made in VB.Net, which acts as an updater, keeping another program updated.
I mean, it downloads an EXE from the internet and replaces it, it doesn't do anything more than that.
Some antivirus detect it as a virus and now even Windows Defender itself removes it from some computers.
How I can avoid this?
I understand it's because my app has "suspicious" behavior, but I need to fix it.
I have read that you can buy a signature for my app, but I would like to solve for free, without buying things.
Thank you.