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