To avoid communication between the hosts from different LAN networks, you will have to enable a firewall zone for each LAN interface configured on your router. Right now, you probably have all the LAN interfaces covered on the same firewall zone (LAN).
To change this default configuration, please enter on your WebUI and follow this path: Network > Firewall > General Settings > Zones. Then, click on the pencil icon to edit the "LAN => WAN" Zones Forwarding. A new window will prompt, look for the covered networks field, and select only one of the LAN interfaces associated with your VLAN/network. For simplicity purposes, I will assume you will choose the home client LAN. To finish, save the configuration.
The following step is to create a new Zone Forwarding for your other LAN network. Add a new LAN "LAN_2" zone and choose to accept all three policies (Input, Output, Forward). Then, select your office client LAN inside the covered networks field, add your WAN zone to allow forward to destination zones, and save the changes.
Now, click on edit the WAN Zone forwardings; in "INTER-ZONE FORWARDING," add your "LAN_2" to the field "Allow forward from source zones" and click on "Save & Apply."
To finish, verify that the hosts from different LAN networks don't reach each other and that you have internet access on both networks.
I hope this information helps to solve your query. I will keep an eye on your comments.