how can i know if my modem supports 5GHz band or not

To connect modem to the network press reset key for 30 seconds. Go to your computer or phone and open browser and type 192.168.1.1 in the top url bar. It will ask for your login information (default username and password is admin for both). Then it will open setup page. Just follow the instructions and your router will get connect to the internet. Once it get configured you will see modem dashboard. Then go to wireless tab and you can see the settings of 5 ghz.

For that , you can check your modem's manual or we can also check by login into the modem's settings
and for that we need to follow some steps:-
> connect your computer with the modem's network
> open the web browser and type the default ip address(for example:- 192.168.0.1 or 10.0.0.1 or 192.168.1.1) and then press enter
> enter the login information for your modem (you can check that info at the back side of your modem itself)
> once you are successfully logged in
> then go to the wireless tab , and from there you can if its showing you the settings for the 5GHZ band or not.