You can only reset an iPhone without the passcode and without a computer if Find My iPhone is enabled and you can access your Apple ID
Go to iCloud.com/find on another phone or tablet
Sign in with your Apple ID
Select your iPhone
Tap Erase iPhone
Confirm the erase
If you cannot access your Apple ID, use the Apple ID account recovery process
If Find My iPhone is not enabled, you will need a computer or Apple Support to erase the device
After the reset, you will need the Apple ID and password previously used on the iPhone to activate it again