@PinPale 

You can always use Address Reservation option. Then, the router will always assign the same, static LAN IP to the certain devices. That's how you usually deal with that and bypass the DHCP for IoT devices.

uPnP allows the client (from LAN) to send info about required ports to be open to router, and automatically creates Port Forward Rule if needed. 

I have no idea if/how is it possible to open a port to whole LAN network. I guess we would have to wait for a reply of someone with better knowledge. Sorry.

  @PinPale 

Google translates that as

Ports that may need to be opened on the router

In order for the smoke detectors to communicate with the cloud service, certain ports may need to be open:

UDP port: 6667, 7000, 8001
TCP port: 443, 8886, 6668

Which seems to confirm what I thought, that the detectors are communicating with a service i.e an outgoing connection , in which case no port forwarding is required. The ER706, like most routers will by default allow outgoing connections on all ports.

It is not possible to forward a single port to multiple LAN IP addresses, so if you think about the situation where you have multiple detectors with different IP's, communication cannot be possible in this way. Hence the use of an external cloud service, each detector connects to that service and updates its status, in turn your mobile app connects to the same service and can check each detector status. In addition, I suspect the service can use 'push notification' to cause your phone to be informed when a detector changes status.

I will see if I can find more detailed information from the suppliers website....

  @PinPale 

I use TAPO Wifi Smart bulbs, Wifi Smart plugs and a Wireless Doorbell ( via an H200 wired hub ). They all work pretty well via the Tapo App on my phone & tablet, although there is an occasional delay when, for instance, accessing a smart bulb remotely.

No configuration necessary on my router.