Another option that seems to work just fine if you happen to already have your Epic Games Launcer installed on another drive other than C:.
Create the Epic Games folder in:
C:\Program Files (x86)
So you path will be C:\Program Files (x86)\Epic Games.
Then run the EpicOnlineServics.msi from
[MY INSTALL LOCATION]\Epic Games\Launcher\Portal\Extras\EOS\EpicOnlineServics.msi
This way you don't have to go through reinstalled the laucher to where you don't want it.
yes that works I have the Epic Games installed on my F:\ Game drive I got another 1 TB hard drive besides the C:\ drive , I am keeping my games installed on it, since it free up space on my main C:\ drive for the operating system and stuff besides the games, its a good idea to have another drive since when you get a lot of free games from it that take up a lot of space on their own etc... I also have that .NET 3.5 installed and was getting that error, since I needed to create the path on the C:\ drive for epic games online services once I did that it installed just fine, this advance was the best for that error code for me. that error code of EOS-ERR-1603 was not related to .NET 3.5 of not being installed in my case it was installed and was related to me installing epic games launcher on the F:\ drive and not the C:\ drive, it needs the folder created on the C:\ drive in order to install its new epic online services which I didn't know since that error page talks about having to reinstall .NET 3.5 or install it since again it was already installed which again in this case was the wrong advance to give for this error. this in my case it had to do with the fact I am using another hard drive for games and the installer itself on the another hard drive to free up space on my main C:\ drive reasons otherwise I would of had it on the C: drive if not for that.