Since you have gigabit fibre. The first things I would look at are, QoS, firewall, and WiFi signal strength. QoS or Quality of Service is a setting where certain things are prioritized over another. If your home network has a lot of traffic always flowing over it.
QoS can be great when setup properly but bad when it isn't. You want to prioritize WiFi Calling over pretty much everything. Asus routers are very good at handing WiFi Calling without any special configuration changes.
Make sure your firewall isn't blocking or messing with the ports by closong them and reopwning them when they should stay open or WiFi Calling to work properly.
Your home being a Faraday cage might be interfering with your WiFi signal strength in different areas of your house. Things like microwaves, cordless phones, and other large appliances can cause interference. Lock in the channels and bands for your wireless instead of having them on auto. If you are using a mesh system. Most cheaper mesh systems take a portion of the WiFi bands for the backend of sustaining the mesh pods.
If you are using an ISP provided gateway. Consider seeing if it has a bridge mode setting and purchasing a separate wireless router to manage your WiFi.
