Apart from setting pool address range correctly, it is necessary to adjust a few other things for larger subnet:
- subnet mask of router's own address ... under /ip/address the new subnet mask has to be same as (or shorter than) the one used for pool, i.e. it has to cover all addresses available in pool. E.g. if you're thinking to extend pool to 10.0.0.5-10.0.3.254 (almost the same as 10.0.0.0/22), then router's address needs to be changed to 10.0.0.1/22 (or shorter subnet mask, e.g. /20 or /16)
If you go for second pool (e.g. 192.168.x.0/24), you'll have to add router IP address from that range and add another DHCP-server network config etc. making it a tad more complicated than extending existing subnet.
- DHCP network settings need to match pool ... under /ip/dhcp-server/network ... the address property has to match pool range, e.g. set it to 10.0.0.0/22 (or some such, it can be wider e.g. same as router's subnet mask). Also adjust property netmask, it has to match netmask of router's address (e.g. /20 or whatever you choose to use).
In theory DHCP address pool can overlap other (statically set) addresses, there's mechanizm not to assign address which is already in use. But it is goot custom to avoid overlap by setting address pool so that it doesn't overlap any of statically assigned addresses (including router's own address).