Setting up guest network on t-mobile highspeed home internet

Thank you for the explanations. So when we created two networks on our combined modem/router device previously, they weren't really separate then either because they were still on the same device? We had one network set up on 2.4GHz and one on 5GHz. And it doesn't appear as though our Linksys AC1900 combined modem/router wants to function just as a router. It seemed to work fine connected to the T-mobile home internet to create an internet signal for our computers but our smart devices didn't seem to recognize that there was an internet connection with it (and so didn't work). That was why I was wondering if a standalone, ordinary router would work. Basically, our goal is not to leave our network vulnerable because of our smart light bulbs :). And we also aren't keen on giving random guests our password.