Please split the DHCP issue from the subnet issue.
There is no DHCP needed to use some subnet.
If your ether2 (or bridge, if ether2 is port on the bridge) has 2 IP addresses, both subnets can be used.
E.G. 192.168.10.1/24 , as IP address on ether2 (or bridge) has 192.168.10.1 as gateway for subnet 192.168.10.0/24 network.
E.G. 172.16.0.1/24 , as 2nd IP address on ether2 (or bridge) , has 172.16.0.1 as gateway for any member of subnet 172.16.0.0/24
Now the DHCP issue. If 2 DHCP servers are defined on one interface, ether2 (or bridge, if ether2 is port on bridge), which DHCP will respond?
Same problem with one DHCP sever and multiple pools (scopes). Not easy, even with superscope (Windows)
Sometimes static DHCP entries take always precedence. So you can force a MAC to use a specific address (from the pool or not)
Others will split the LAN in VLANs per subnet, (One or more DHCP servers can serve part of that VLAN subnet, with the correct pool)
Devices from 2 different subnets on the same LAN, need routing to talk to each other. The RB951ui with addresses in both subnets can do that.(if FW rules allow inter-LAN traffic)
By default the NAT rule and "default gateway" is linked to the WAN interface. So ALL LAN networks (or subnets) can access the internet with the same default rules.
i set the nat-route and the DHCP server rule on the 192.168.10.1 range
That is you limiting that NAT rule to only work for that subnet. either extend that NAT rule with the other subnet or remove/alter the limitation
Limitation should be outgoing interface or interface-list. Otherwise it would be activated for LAN-LAN routing