I had this issue with my Dual Sim Nokia Lumia 950. The 5.0ghz wireless would work then disconnect and disappear and no longer be an option to connect to. I fixed this by downloading the windows device recovery tool and updating the firmware on the phone that way. Just follow the instructions and update to the latest firmware VIA the tool with the phone connected to your computer. It worked well for me so far.
https://support.microsoft.com/en-us/help/12379/windows-10-mobile-device-recovery-tool-faq