Bit of a N00B / trying to bridge an existing WiFi LAN

Here's the setup.

I'm renting an office and the landlord provides wifi with internet access. It's been a long time since I was up to speed on the gory details of networking, so some of my terminology may be off. Bear with me!

I think what I want is a bridge and a subnet, but I'm not sure.

What I want to do is have all of the devices in my office be behind a firewall that I control. I want the devices to all be able to see each other, share files and access the internet. I also want our group of devices to be invisible to any other users in the building i.e. they should not be able to see any part of our network. 

I have an R6300v2 with DD-WRT installed. The DD-WRT version is Kong Mod for NETGEAR R6300v2 (2017-03-21). I do not have access to the landlord's router, other than the wifi password.

All suggestions welcome!