There are two types of iron. Clear water iron is what I'm familiar with. Clear water iron does not show up until it's allowed to come into contact with air. Then it can slowly settle out in a glass of water, bath tub etc. Clear water iron must be oxidized before it can be removed by an iron filter. These in line filters you have will not remove clear water iron as you have probably already discovered.
A "Real" iron filter looks very similar to a Water Softener. It has different resin specially designed to remove iron. Some use air and some use Potassium Permangenate to oxidize the iron. Bleach will also work, but I prefer to not use bleach.
Iron filters can be bought locally or on line. If you are going to install it yourself, you can get a good one in the range of 5 to 7 hundred dollars. I sell them.
I hear a lot of softener companies say that their water softener will remove up to 10 parts per million of iron. I have tried it at my own homes over the years and have had no success at all. Even using fine mesh resin. An iron filter should be installed in front of the softener to save wear and tear on the softeners resin.
First, have your water tested to see which of the two types of iron you have and how much in PPM.