OP - It sounds very much like your router does not support NAT Loopback. NAT Loopback allows connection to your own external WAN IP address from inside your private network. Check your router documentation to see if there's a setting. Other than that, you will need to switch addresses depending on which network you're on.
Take a look at my Honeywell VKP app manual, as it explains this on the second last page. I don't know if Texecom mention this anywhere, but it's a very common issue.
https://www.sm-alarms.co.uk/manuals/selfmon_iosvkp_v2.pdf