I have comcast and I have dual bands. Comcast knows that most smart appliances require and work best on 2.4. They cannot give you 5 gHz without providing 2.4. what Comcast does do for user convenience is bridge to two bands and let the devices work out which is best. This does not normally work, as most smart devices are simply not that smart. So, whoever at Comcast told you that is either being dishonest or us unknowledgeable. You can access your router settings and separate the two bands - or you can have a technician do it if you don't feel comfortable.
The router settings should look like this - if you don't see both networks listed separately, tap on the "disable" icon, then choose a name an password for the 2.4 network (it can be the same).
