Impossible to connect smart plugs to my WIFI

[#1]

Can anyone help with this? I have some new smart plugs, I have two different brands, and I've tired two different apps. 5ghz is switched off on my router. They still don't work. The error message I get in the SmartLife app is that they can't connect to the router (attached screen shot). The password is correct, I've tried factory resets, restarting the router, switching channels, the standard set up advice, all the usual things. My router is a few years old. I tried them at someone else's house, they work fine, immediately. So nothing is wrong with the apps or the plugs, but my router is somehow stopping them from working. Can anyone help me out here? I'd really appreciate it. It's driving me insane. Any more info you need just ask and I'll send it. Thank you!

[#2]

Tried activating 5ghz?
Secondly, try disabling security on your router temporarily; does it then connect?; if
so that's the issue.
Has plug got a WPS button?
Tried connecting via that method?

[#3]

Thank you, much appreciated!! I'll try these options and let you know.

[#4]

I've reactivated 5ghz, this didn't make any difference. Also disabling security and firewall, no difference. And I tried the WPS button to connect, also no joy. If you have any other ideas, happy to try!

Thank you!

[#5]

This echoes an issue that I've had with my laptop, and a few others here have had: there just seems to be compatibility issues between some wifi devices / access points. I ended up getting a USB wifi dongle for the laptop.

Also, a few months ago, I had problems trying to get a smart-heater to connect properly to someone's wifi. 

They are all built to the same specs, but the specs allow you to set so many different options (go look in the windows setting for a laptop's wifi!), sometimes things just won't work 😞