your description is still not clear enough about the topology, the router is having LAN ip address, and if you want to access it you should have to connect over public internet. the problem is how...

You mean after upgrading the firmware, the default password for the router's wireless changed, am I right? Suggest you to contact support@tp-link.com and then tell them which firmware version you are...

which ip address were you ping, and how you connect all the equipment, i mean which device are connected to the other side of the LAN cable while using this router. Generally, you connect the...

can't downgrade firmware from one hardware version to another

could have a try with the "Access Control" function

841 don't support facebook wifi, may be in the future will support ~