I've checked the documentation and changed the RTPengine setting to: interface = 192.168.1.75!62.2x.2xx.xxx as stated https://rtpengine.readthedocs.io/en/latest/rtpengine.html#advertised-address. However, it still doesn't work.
If you think it might be something else, I'm happy to test further. If not, thank you for everything. Thank you Henning,
Fernando Lopes