Is it possible that you set them up under two different accounts? Normally speaking, once you’ve set up one speaker, you would use the “add a device” function inside the controller, rather than creating a new account completely.

It’s also possible that they could be on different network ‘subnets’, in which case they wouldn’t be able to “see” each other. If this is a possibility, we might be able to help more if you were to describe your network, including device that the controller is on, router and type of router, and how your Sonos is connected (wired? unwired? BOOST? Bridge?) and your version of the Sonos controller (S1? S2? which version number of the controller?)

FWIW, 5G is a frequency your phone uses to connect to the phone service. There are wifi frequencies at 2.4Ghz, and 5 Ghz, but neither is called 5G. 

It is also possible that the issue is caused by your phone connecting to the 5GHz band and the Beam being on 2.4GHz.  But only if your router doesn’t allow traffic between the two bands, which is pretty stupid but not unknown.

The simplest test would be to turn off the 5GHz band and see if all is visible.

It is more likely that one of Bruce’s suggestions is the cause.

All the home theatre players can only connect to a 2.4GHz WiFi. Their 5GHz radio is reserved for connecting directly to surrounds or Subs.