you need to log into your home modem to do the setting, usually it start with 168.192.1.1 and so on , it depend on what model and Internet provider you got, you also need a user name and pass word to log in. you probably find that at the back of you modem
2.4ghz is usually enabled on all routers, unless you turned it off. So you door cam should connect to your router on 2.4ghz if available and no change required.
If you turned off 2.4ghz then turn it back on again.
You didn't tell me the make of the door bell? It may just be issue with door bell