@Katetamm 

Hi,

Have you already tried what happens if you switch off "Use Private Address" or "Private Wi-Fi Address" on the iPhone for the wireless network of your TL-MR6400?

Are you using the Access Control feature of the TL-MR6400?

On your laptop, do you mean instead of the webpage you intended to open you are shown a webpage from the TL-MR6400 router on which it asks you for the SIM card pin? 

  @woozle When I switched to Use Private Address on my Iphone, I was able to get online, but just on that phone. I could not on my laptop. What I was trying to do on the laptop was get through to http://tplinkmodem.net/ but the page would not come up. I connected by lan from the modem to the laptop. I was then able to get the http://tplinkmodem.net/ page, and it wanted me to unlock the sim - which had been unlocked. Everything on the wifi was now working. But as soon as I moved the modem back were it should be - unplugging it, I was back to no internet, and not being able to get back on to http://tplinkmodem.net/. Eventually, I solved it! There is are two tick boxes on that first page of the Sim pin one has something like 'automatically authorise' - I can't get back in to check. Essentially, the wrong thing was ticked, and it was locking the sim again everytime I moved it. 

Thanks for you help.