Look into Pi-hole. It’s an easy-to-setup DNS server which can run on a Raspberry Pi (or a Linux desktop/server if you have one.) You can then set your devices’ DNS servers to the local address where the Pi-hole is running. Since it would be running on your local network, any requests to it shouldn’t go through your ISP in the first place. I’d still recommend getting your own router anyways because this kind of ISP fuckery is more common than you’d expect. Plus, your exact configurations follow you anywhere you move. If you do end up getting one, set the local DNS server in the DHCP settings of your router to avoid having to set it on each device.