Hello! I'm aware it's not really intuitive. In the next update, you have to actually EQUIP the items into the fishing rod and you see the stats the moment you're fishing.
How does it work right now?
There are 3 values; Strength, Speed, and Seconds.
Strength improves how much "damage" you do to the fish BY REELING. Moving the mouse around the circle thingy where the arrow points.
Speed improves how much "damage" you do AUTOMATICALLY. It is not a big change, but with a couple of improvements and items, you can get a really nice boost.
Seconds are the seconds you get to catch a fish.
You DON'T HAVE to do anything. Once the items are in the INVENTORY, they apply the boost. (Read the item description)
Also, you can use the levels you get from all activities to upgrade fishing. Check the book at the right corner of the screen.
I hope this information works, at least for now.