When I used to do this, I just bought a $60 router to hang off the main router, and used that router to connect the devices I watched baseball on, and left the main router untouched for the rest of the household.
This is definitely a solid option. Depending on what you're connecting, you can also manually set the DNS on each device. That requires a bit more understanding so you make sure you aren't giving out an unusable address or setting it to another address currently in use. nfotiu's suggestion helps sidestep both of those, as you just need to set the DHCP settings to hand out addresses between say, 10.30.10.10 and 10.30.10.30, with 10.30.10.1 as the gateway, and the DNS configured for smartDNS. Leave the router's WAN connection as DHCP (obtain an address automatically) and you're good to go.
Happy to assist by looking at screenshots and offering suggestions if you get stuck on a point somewhere, regardless of which path you take. Just make sure there is no personally identifiable info if you post them here instead of a PM. There shouldn't be, but Verizon was setting broadcast WiFi to people's street address with the home phone number for the password, once upon a time. They've stopped, but depending on how old that setup is, they may have grandfathered it along.