The basic BT Home Hub 5a is with stock firmware a locked bog standard router but if installed with the power or OpenWRT/LEDE then it’s capable of a lot more!
To do this it will require you to open up your router, solder a few wires to places and then use a serial USB cable with the help of putty (terminal program) and a few other programs to dump and inject the firmware but once done the router will be unlocked to any ISP and open to more capability.
Take a look at the OpenWRT website here for more detail.
There’s a lot of packages that can be installed and you can customise to your own needs and wishes.
My personal preference is the ability to set up a VPN which will allow you to connect to your home network when you’re out an about or away from home.
Make sure you read the how to guide fully and allow yourself enough time and patience to do.
Once done you won’t ever go back to a stock router ever!!!
The Hub 5a isn’t the only router that can be opened up to the world of OpenWRT and LEDE so take a look at their website and you might have a router that is supported.
Each router has a different method of installing the firmware on and some are more complicated than others but on the other note, there’s some routers that can be flashed with simply a usb stick.