Either you are lucky or your browser usage is very less.
So you are getting ads in your browser? No Internet connection can insert ads in your browser. The way ads are managed are as follows:
1. Through digital advertisement.
2. Through your browsing habits based on your search pattern. For example, if you search for a car, existing car advertisers will be routed to your browser next time you log in. Similarly if you search for an Internet connection, existing internet connectivity ads will be routed to you.
Most of this is done by Google or other search engines. The internet connectivity company has nothing to do with it. If the company wants you to see an ad, they have to pay Google and Google will show the ad. The only way entities can bypass this is to download some application on your computer that loads up when you switch on.
Some companies, hotels, and other institutions allow you to connect after asking for a temporary id and password. At that moment they put up their name and maybe some form of ad as the first login screen. But once you pass the login, the browser takes over. If you see some ad that comes up by itself, it will be because of a malware, a cookie, some virus, or your search habit. If you go to some dark sites, ads will be thrust upon you using a new tab. But, this also happens only when you click on a link. None of this has anything to do with the company that gives you the Internet connection.
Next time you travel by train, check into the Internet using RailTel. As I said before they will ask for your phone number and an OTP where Railtel will be 'advertised'. But once connected, they dont show up in your face anymore. None of these purposely slow your connectivity speed. Some time ago certain companies such as AirTel were slowing down your connection by throttling the speed based on you paying and/or usage pattern. But they were all rapped by the courts, and all that is in the past now.
BNSL gives you a user id and unique password that you use for the first time. After that they never ask you for anything. They connect automatically. It the same with all companies that give you connectivity.
Run some form of cleaner to remove malware and viruses literally everyday. Also clear your browser cache regularly. You will not see any ad. If you do see an ad after all that, there is something wrong with your computer and/or browser.
Cheers