DDNS is exactly what would allow you to maintain access to your local network If you had a dynamic public IP address. However, it seems that your ISP is using Carrier – grade NAT itself, and, in order for you to gain access to your network, your ISP would have to perform port forwarding on its router, which is highly unlikely. Unfortunately, you will not be able to access your local network via WAN connection without public IP address directly.
I would advise you to look for VPN solutions. One such example could be ZeroTier as it does not require for any device to have a public IP to establish a VPN tunnel. It is a downloadable package, so before being able to configure it on your device, you have to download it. You can do it by accessing your router’s WebUI, going to Services -> Package manager -> Packages, and pressing „+“ under Actions column next to ZeroTier. A configuration example is provided in this website: https://wiki.teltonika-networks.com/view/ZeroTier_Configuration.
If you do not want to use third party solutions, you can check Teltonika’s Remote Management System (RMS). This system allows you to get remote access to your router via WebUI or CLI without the public IP. More about RMS can be found in the following website: https://wiki.teltonika-networks.com/view/RMS.