Not sure. The printer works OK when it is connected direclty to your PC? The driver has been installed? There is an instruction and you may follow it to try again....

After power cut-off, you have to reconfigure WR740N to access internet? Did you try to restart it to see if it works? The main router still works after power cut-off? Did you fix a channel for 2.4GHz...

DNS problem? So Wi-Fi still shows connected on your wireless clients and there is IP address? Can you try to change DNS to 8.8.8.8 manually to see if it will fix the problem?

How did you test the speed? Can you reset ER5120 to factory defaults and reconfigure it to work with your new modem? Later, connect just one computer to ER5120 and test the speed again.

Model Number? Access control can fulfill it but you have to create proper rules. Are you trying to stop internet on a specific PC or all of them? http://www.tp-link.com/en/faq-359.html

Did you refer to printer controller utility? The printer is connected to USB port of C2? http://www.tp-link.com/common/compatible/print-server/