For Araknis Routers this setting is actually under Settings > Firewall on the router.
For other routers the settings can vary. Some have it on always with no control. Some it will be in the Firewall or VPN settings of the router. There are some (ISP provided) that do not have it on and you cannot turn it on. One of the biggest reasons for bridged modem/router combos or providing your own standalone modem.

The next thing to remember with Wi-Fi calling is it can all be different for every phone and cell provider. They want to make sure your phone can complete a phone call and it will use the best option to do so. Most often it will trust the cell signal to do this even when Wi-Fi calling is showing active on your phone.

The best way to test Wi-Fi calling for the customer is to actually turn off cell service on the phone. You can accomplish this in a few different ways. I usually just recommend enabling airplane mode and then turn Wi-Fi back on. Walk around while on a phone call. This is a great way to test Wi-Fi calling but also overall Wi-Fi coverage. Mainly because this is a real-time test(no buffering). You will quickly identify roaming or dead-zones.

As Nima mentioned make sure your phones are up to date on all software releases. I just received a recent update for my Google Pixel 3a and I noticed that I have new icon when my phone is using Wi-Fi calling. Also remember that Wi-Fi calling only supports phones calls at this time, it does not help with text messaging(I see that misconception quite often).