I'm not very familiar with network setup and have the following situation and desire.
I have a main router and via DHCP it assigns dynamic addresses to guest devices and some static addresses to some specific devices.
Now I'd like to add another router, mainly to improve WiFi coverage. This 2nd router should be also usable as a standalone travel router for the caravan. Therefore it would be desirable that it also has DHCP activated and assigns the same static IPs for specified clients as the main router. The dynamic IP range it uses may differ from the main router.
Is that possible and what do I have to keep in mind?
I'd recommend using two different devices for this... one of home, one for your caravan. Typically speaking, the travel routers have lower wifi performance than APs designed for home/business installation.
This is possible in isolation (i.e. you could use the same DHCP reservation information for each of the routers, but only one DHCP server can be active at a time). It is not possible (at least not without really sophistocated network infrastructure like you'd see in enterprise environments) to run multiple DHCP servers on the same network without causing major issues.
What I'd recommend is 2 devices... one for home, one for the road. However, if you want to do this all with one device, you need to change the configuration -- home = dumb AP mode, road = router mode. If you happen to do this with a travel router, many of those have mode switches on them, so you can configure the device such that the position of the mode switch determines the running configuration.