Did you use Internet Explorer or any other browser to upgrade the firmware? I suggest you to try with Internet Explorer. In addition, I'd like to remind you that: use the wired connection instead of...

As far as I am concerned, TD-8616 does not support PPPoA, it seems it only supports the protocol end with E, for example, PPPoE, IPoE...

Have you tried to manually fill in the correct APN and dial number within the 3G part? I catch a massage in the log: 1st day 00:21:41 3G/4G NOTICE 3G Modem connecting error, please check the dial...

How about disabling the Mac Filtering temporarily to have a test whether TL-WR740N will function or not? By the way, is the TL-WR740N wired connected to 8961ND as well? I heard that 740N cannot WDS...

As far as I'm concerned, it does not matter whether they are connected to the switch or wireless router, the switch does not have NAT, so it seems like to add more Ethernet ports on the wireless...

It seems you have got an IP address with the 3G part. when the connection is down, have you ever noticed whether the IP address is still there? if the IP address is still there, perhaps it is...