Probably that is a slow updating dns server you use (provlbably coming from the ISP), next time try to use one of the public ones, like google 8.8.8.8.
Yes it will work only inside home LAN.
If you want to have external IP you need to create a port mapping on your router and enable external access in Settings - Access (it will do the port validation for you).
but can I several device?
Yes you can we have no limitation (if you are not too greedy and start running 100s of devices under one account)
All requests are going through the router to the internet so we see your requests are coming from your router’s public IP address (we show it on your device page) but it does not mean anyone can actually connect to your router from outside.
For that you need to create a port mapping and tell the device that this is what you want (enable Settings - Access).