How to remove a Remote Management Software from my iPhone 6 plus?

You can try a DFU restore:

• Note that you must have a computer running Windows 7 or later, or Mac OS 10.9.5 or later
• Install the latest version of iTunes on your computer if it isn't there already
• Reboot your computer
• Disable any antivirus and firewalls
• Launch iTunes
• Connect the device cable to a USB port on the computer (not to a USB hub), but DO NOT CONNECT TO THE DEVICE YET
• Press and hold the HOME and SLEEP buttons at the same time for 8 seconds, until the screen turns black, then release the SLEEP button but continue holding the HOME button. If an Apple logo appears while holding both buttons you held for too long; start over.
• While still holding the HOME button connect the other end of the cable to the device
• You should see a message in iTunes saying a phone in Recovery mode has been detected (it may take up to a minute for the message to appear). When you see that message release the HOME button and the device should restore. When asked DO NOT restore a backup; set up as a new phone.

If this does not remove that message return the phone to the seller, who should have checked this before selling it. It means that the phone's ownership is stored in Apple's activation database, and it may be a stolen phone.

The seller sold you a stolen phone that is linked to a corporate MDM through the Device Enrollment Program. Return it and get your money back or contact the corporation it's locked to and see if they would like their stolen property back.

This is intended for a business or school that manages phones, but it will give you an idea of what is going on: Use Device Enrollment - Apple Support

This is a different problem from the previous phone. That was to remove Activation Lock. This is because ThyssenKrupp issued the phone to an employee using Apple's Device Enrollment Program, which assures that the phone will always be under device management by them unless ThyssenKrupp removes it from device management. The fact that the phone is owned by the company is recorded in Apple's database, so when you try to activate it that message will be displayed. There is no way around this unless the company removes it from device management. And the fact that you have attempted to activate it is logged by the company's management database, so they know that someone is trying to use it.

As you are sure this is a reputable seller return it to them.

iman_montreal wrote:

What is my best option now?

Your best option is to return it for a refund. If the seller refuses, then file a claim with eBay and get your money back. They misrepresented the state of the phone.

You can not bypass the lock on a device that's in the DEP. The seller's reputation is irrelevant. They sold you a phone that belongs to a corporation. They may not have known it was locked to the DEP when they acquired it, but that doesn't negate the facts.

Return the phone. You can't use it. If they are a top-rated seller they should gladly refund your money.

Best of luck, no matter which solution comes home.