Networking

As the title alludes, I have two devices that I would like to connect to my LAN (a TC - wired; and a phone - wireless), but I would want them to be connected to a separate subnet, so they wouldn't communicate with the rest of my LAN.

Currently I have those two wirelessly connected via my router through a guest network, which automatically assigns them on a different subnet, but the router (provided by my ISP) is pretty crap, and the wifi networks either drop, or have bandwidth issues quite frequently. Unfortunately, due to the router not being the best, it doesn't allow me to assign IPs for those two devices that are outside the LAN subnet.

I would like to be able to connect the TC via ethernet instead, to guarantee the best connection, and perhaps link the phone with a separate wifi network, or perhaps just use data connection, thus taking it out of the LAN completely.

To achieve this I was thinking of getting a second router (second hand perhaps), flash OpenWRT on it, and use it to connect just those two devices.

Another option I've heard of would be to use a managed switch which allows me to set it up to connect devices on another IP class.

Could anyone let me know what would be the best and affordable option for this purpose?

Thank you in advance!