TL-WR642G should be connected to one of LAN ports on your modem. Make sure your modem is working firstly and connect tplink to any one of 4 Ethernet ports on TG605s.

this is strange~ have you rebooted your smart phone? try to forget the wifi name and reconnect to it?

it's unnecessary to release two same problems here:)

how about if you connect to your main modem directly? Does it still meet the same issue? Seems the WAN connection is not stable. Make sure the ISP connection is stable.

we have to select Only the PCs listed can browse the built-in web pages to perform Administrator tasks, then I can click on Set button. TPlink should make some changes on this and can't add more....

it seems you need a wireless router which supports various Wireless Application Protocol. However, I don't know which device is available in UK.