I see two ways you could do this:
- Disable randomized MAC for that specific network, you can do this under the
Network Details > Privacy of that network, then just assign a static IP from the router.
Based on your post, it seems you own the router and have admin privileges to it, else you wouldn't be able to assign static IP addresses through it, so I see no reason why you wouldn't trust your own device with your phone's MAC address.
- If you still want to keep the randomized MAC address for your phone, you could try in the router setting to decrease the DHCP range
Let's say your lan ip range is 192.168.0.0/24, just give your DHCP access to all IP addresses between 192.168.0.100 to 192.168.0.200, then you keep everything from 192.168.0.2 to 192.168.0.99 for the static ip addresses.
You can set through the phone settings under the Network Details > Edit > Advanced Options > IP Settings > Static and then Network Details > Edit > Advanced Options > IP Address > set 192.168.0.XX (XX being between 2-99)