What's the operating system of your PC? Check the IP address obtained with limited internet access. If it is 169.254..., try to assign a static IP. If you don't know what you should put in, refer to...

Have you tried different channels like 1, 6 or 11? Change channel width to 20MHz and test if it is more stable.

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...