How about changing the sim card? Or does the error still exists when no sim card inserted? If yes, it seems to be a hardware issue. Get a MR200 V2, or contact tplink technical support

What's the model number? What's ISP update file? You mean the firmware for the product?

you may contact sales support http://nl.tp-link.com/contact.html

We cannot find out until login the modem to check. Or you may check with ISP because the firmware needs to be passed by ISP to make it compatible, and whenever there is newer firmware, just reboot...

Yes it's OK to plug the 941HP in the VR600 to work as a secondary router. Or if your 941HP is version, you can set it repeater, and move the 941HP to another room to extend signal.

The smart plug can be managed remotely when you are not home, not on the same WiFi, as long as remote control is enabled on the KASA app and you've login with your cloud account. If it does not work...