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Note that this fix will only work for using your friends list and going online in games, eshop bans do not have a fix yet as far as I know
What you need: An unbanned 3ds with decrypt9
OR a fresh LocalFriendCodeSeed_B from another source
1. Backup your banned 3ds's LocalFriendCodeSeed_B by dumping with Decrypt9 and saving on your comp
2. Dump LocalFriendCodeSeed_B from unbanned 3ds with Decrypt9
3. tranfer unbanned LocalFriendCodeSeed_B to banned 3ds's sd card
4. inject fresh LocalFriendCodeSeed_B into banned 3ds with Decrypt9
5.reboot
6.check friends list, it should say online again
7.check eshop (not sure if necessary)
8.then go online in game.
pic for proof