You could go to
https://icloud.com/, then to
Find iPhone. Then click on
All Devices at the top and select the device you want to erase.
Then, it should give you an option to erase your device.
Of course, be careful, because a mistake will result in the wrong device getting erased.
Also, once the erasing is done, you will need to remove the device from the account by selecting it again and choosing
Remove from Account, otherwise it will be locked to your account.