Wi-Fi Calling stopped working on iPhone 14 Pro

I'm really hoping I can get to the bottom of this issue as I've been tearing my hair out over the past couple of weeks with no resolution. 

Affected Device(s): Apple IPhone 14 Pro running on 18.2 iOS (as of 12th December 2024)

Wi-Fi setup: Router issued from a local Fibre network company wired directly into a TP-Link Archer AX-10 providing both a 2.4Ghz & 5Ghz Network and QoS High Priority Client enabled for the above iPhone device.

Problem: Since updating iOS to its latest version I have been experiencing issues making and receiving calls when on Wi-Fi with Wi-Fi Calling enabled – the indicator in the status bar signals that the feature is on when connected to a Wi-Fi network. As I live in an area with low signal coverage I rely on Wi-Fi calling and have done for the past four years with success, but all of a sudden it stopped working.

Diagnostics: 

iPhone Settings: So far, I've tried all of the iPhone settings such as resetting Network Settings & Network-related Settings, turning off and on Airplane Mode, toggling Mobile Data, toggling Network Selection, removing VPN profiles, ensuring Location Services & System Services are correctly configured, checking the carrier profile and APN Setting and, of course, turning the device on and off with the SIM card removed. When speaking with the Senior Apple Support team they did mention that I'm missing additional menus below the Wi-Fi calling On/Off toggle page i.e. Update Emergency Address & Add Wi-Fi Calling for other Devices and that the carrier needs to enable something on the backend – I'm unsure how this would have been removed randomly unless there was a conflict with the most recent iOS update.

Home Network Configuration: With the help of Mr GPT, I set up some firewall port forwarding (500, 4500, 123, 5223), a few other tweaks to ensure there weren't any conflicts and the IP ranges of each router and its devices did not conflict with each other. I did also reach out to my Internet Service Provider and they assured me that this was unnecessary as it work out-of-the-box as it did just a couple of weeks back, so I reverted everything back.

Once I finished speaking with my ISP and Apple I then moved on to speak with Three Support and as usual they were just reading instructions on a sheet, the stuff I had already mentioned that I had tried over and over again for the past 24 hours and that Apple required me to speak with my carrier (Three) to resolve it. Three's answer was to either go back to Apple or to my ISP to resolve. As you can imagine, I'm a bit frustrated after spending a full day working on this with various support teams with still no resolution.

Any help, pointers or additional diagnostics would greatly help!

Thanks in advance!