Modem tplink w9960 fails to get ip unless I connect zte h168n first

Hello, currently am facing a strange problem. the problem is when I need to connect tp-link w9960 with OpenWRT it fails to get IP (Negotiation failed or connection attempt failed) but when I connect ZTE h168n modem it connects smoothly and after that, I reconnect the tp-link modem and it works just fine. I need to know why the tp-link modem is not connected smoothly and I should connect ZTE first?
could it be an ISP problem? or a Hardware problem?