What does it has anything to do with car engine? How do you connect the M7350?

The device is not compatible with AT&T or others in the US. May try to manually put in the information from AT&T. https://www.tp-link.com/en/faq-794.html

Is that error showing on your TV? If so, it's better to contact the manufacturer of the TV to get some advice. I assume all wireless devices work fine beside the TV, right? Since it's been working on...

You mean LEDs are off whe it's using specific operation mode? Is it in DSL modem or wireless router mode?

Do you mean address reservation? The limit for that is 32

If the setup is correct, power off the modem, turn it back on in about 10-20 minutes. Sometimes it does sync after waiting. Double check all the info, including ppp username/password, VPI, VCI with...