Can 4G be used on a 3G phone if 4G bands are supported? I have a 3G phone but it supports the 4G band (1800 MHz) which is available in my area. Is there any way I can enable my phone to support 4G?

I went to *#*#4636#*#*, and there is an LTE option, but I cannot select it. Each time I select it, the option changes to WCDMA Only or WCDMA Preferred.
Is your phone called the Micromax a350 Canvas Knight? If so, I'm sorry, but your phone DOESN'T support 4G, according to the specs sheet found on gsmarena.

Don't be excited to see the LTE in the *#*#4636#*#* options, because that menu lists all the possible bands there is, although your phone might not support all or some of them.

The maximum you can connect to is 3G, you therefore can't use the 4G in your area. The limitation is not software-related. It is hardware-related. The hardware component responsible for supporting 4G band isn't in your phone.