Oh I didn't know thought about this solution, but since the Netatmo devices are using the WAX206 at 192.168.5.0/24 as gateway, I think that if I'll the DHCP from the r4s router at 192.168.1.0/24 they will expect an IP in the .1 subnet. I have no idea, I've never used the option 121, only the most commons 6/12.
Hosts / nodes / endpoints are part of the routed network and run code to support routing. In a typical network with hosts connected to an "access" network, only a default route is needed. Outbound ip communications are (in general) a case of - self || directly connected unicast || not directly connected unicast || multicast || broadcast - . So for the case "not directly connected unicast", if a host is in a network (LAN/subnet) that has additional gateways to reach some networks, the route table on the host needs to be be populated for it. Your change is a partial/full workaround - but is it fully correct and does every device work with it?
...ehm yes the problem is just, how? 
they don't have any settings.
I thought also this, but since all the iot devices are wireless 2.4ghz devices, I preferred to create a 2.4GHz WLAN assigned to a new network on a different subnet, in the end (I thought) it should be almost the same as have a VLAN but now I think this is not that easy... but I saved one ethernet port on the Access Point and I also don't like tagged VLANs.
Anyway thanks for the help, but I think we have to continue in one thread only to avoid confusion. This one, or this Netatmo HomeKit native devices, not able to update iOS Home app when isolated (but work online/using Netamo app) - #9 by giuliomagnifico 