Make sure you have the correct region set under wireless settings.

If you don't have any 802.11b and/or 802.11g only clients change wireless mode to n, when wireless mode is set to n 802.11b and/or 802.11g only clients won't be able to connect to the wireless.

802.11AC clients will connect to 802.11N wireless APs just fine.

Reducing channel width to 20MHz may slightly improve range. 20MHz generally has more than sufficient bandwidth for ADSL2+ but if streaming/transferring data to/from/between wireless clients 20MHz may reduce wireless throughput.

Try a few different wireless channels, it's a good idea to try channels 1, 6 and 11 at the very least.

If your area has a lot of 2.4GHz wireless networks 20MHz is highly recommended and try every channel as one channel or channels may provider better range than the other channel(s).

The TD-W8970's wireless AP just isn't very powerful and replacing antennas with the aftermarket 8dBi TP-LINK antennas only marginally increased performance in my experience.

More modern 802.11AC wireless APs have higher wireless power and generally better range.

You have to know that connection between router and phone is two-way and in your case phone has too small antennas inside over the box