1st.. Just for clarification, when using wifi calling it does NOT use the cellular network one iota. Your phone is using the high speed internet wifi router as your tower. Now with that being said. Many wireless routers broadcast at multiple spectrums 2.4ghz and 5ghz.
As a Network engineer for a competitor of your cell company I would ask you if the wifi on your phone works else where aka work, Friends house etc.. If it's still an issue then yes thier may be a software issue on the device/s.
Now if it works fine elsewhere then the issue very well may be with the wireless router. Simplest way is to connect to the 2.4ghz if you connect to the 5ghz automatically and test or vice versa.
If that fails, then you will need to find out 2 things from your ISP. 1) What are your download / upload speeds. If it less then 10mb download and 5mb upload if may not work. 2) Is thier any update that needs to be applied to the router. The new Galaxy s10 line has new features to handle more advanced routers.
Finally, if all else fails. Inquire with sprint about a 4g Network Extender. The physical extenders do a much better job than wifi calling software on all phones.
Hope this helps
