When connecting using Ubuntu, Android or iPhone IOS, they are all connected wirelessly, right? Wireless connection is stable enough？ Cannot figure out if it is VPN connection issue or wireless...

Umm, you didn't mention the method 3? That is not connecting via WPS and it should not cause any problem. Did you contact your printer's technical support?

Yes, you can. Download the Archer C7_V2_V3_141110 one. It improves DLNA compatibility, which should help to solve your problem.

the extender is above the sitting room where the devices are. Did you check signal strength on those devices? When they are in the same room as the extender, how about stability of internet...

W8960 V3 is very old and we cannot expect a newer firmware. You can try to load the old firmware back to see if the problem will disappear.

Your WA850RE is running in latest firmware? What's the firmware version of your TL-WR542G? Can you provide the screenshot of the problem you described?