New gen router can combine 2.4 & 5ghz together. But there are unstable switching problems. I own a Mercusys MR70X that can do that and it is called "Smart connect". Click that and it merges 2.4 & 5ghz under 1 SSID.

I do believe that some routers given by ISP would have locked some features for some reason or another.

It is also true some phones, pc wifi or wifi devices cannot connect to certain routers because the technology/software design is either too new or too old.

Recently i came across my customer office that uses Maxis wireless broadband for the office via maxis provided router. After all their pc upgraded to Windows 10 certain websites cannot view. End up found the problem is with router firmware too outdated to support new internet protocols. And maxis had to change the router for them.