How can I use other materials to upgrade gear in Marvel Future Fight?

I'm playing Marvel Future Fight for iOS and I am with my heroes at level 18. I have many other materials to perform the upgrade gear but none of them appears in the boxes.
eg: I have 6 High Strength Fiber, which says: "The basic materials to upgrade Combat type Hero Gear".
But all these other materials are not available for me to do this upgrade, even though I try to upgrade Captain America, a Combat Hero.
My question
Is there any level limitation or anything else that would prevent me from using these materials to upgrade? I can't find this answer anywhere, even in the game manual.
Best Answer
Upgrading equipment past +4 requires other materials besides upgrade gear.
Pictures about "How can I use other materials to upgrade gear in Marvel Future Fight?"



How do you upgrade your custom gear in future fight?
by touching the icon next to the hero's portrait during battle. An icon is displayed on Custom Gear with useful effects, such as Guard Break Immunity, Invincibility, Increase damage for 1 attack, etc. This Custom Gear is very useful, so check the icon to manage your Custom Gear conveniently!How do I upgrade my character in Marvel future fight?
To improve a character's rank, you must invest both gold and biometrics. The latter is the same items that you used for unlocking the said character. By collecting more of these items, you can use them to increase their rank.How do you get free diamonds in Marvel future fight?
[Marvel Future Fight] How to Upgrade Hero Gears
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