What adsl modem were you using prior to using the Archer D5? What ATM configuration and WAN service were you using to route your block of IPs with your original adsl modem? vpi? vci? PPPoE? PPPoA?...

Good choice securing your data with IPSec.

If you wish to use a dhcp relay or ip helper-address always place those commands on a L2 switch that supports those features, not on the router.

I have a question. Why are you using L2TP to accomplish the virtual private network in your diagram? L2TP offers no encryption and no security what so ever.

Yes your device TL-WR702N can connect to a wireless network by bridging or repeating but this would require the public wireless network to support bridging or repeating. Even still your connection...

You have a very interesting trouble. Just out of curiosity can you please check the mac addresses of each of the network cards in your PC and Raspberry, the wifi card in your tablet, I-pad and sony...