If you connect the old router back, do you have any disconnection issue? Upgrade the latest firmware, reset and reconfigure your router. http://www.tp-link.com/en/download/TL-WR841N_V9.html#Firmware

Is it an apple account issue when you try to install or update APP? Ipad mini is connected to WR841N's Wi-Fi network and you can access internet?

If you doubt it, why not confirm with the person who contacts you? Did you read this page? http://www.tp-link.in/support/rma/ I've no idea about the details.

Why do you want to reset it? Is it still under warranty?

No way to change Chinese firmware of TP-LINK products sold in Mainland China to English firmware. If you need technical advice, contact fae@tp-link.com.cn.

Did you change IP address of WR743? As it works as an access point, DHCP should be disabled. To login its web interface, disconnect it from existing network first. Connect your PC to its LAN port,...