Hahaha, I am NOT a bell training expert. But we did train Nikko to ring a bell when he had to pee. This worked to a point... he would ring his bell every time he needed to pee/poo, and he never had accidents indoors after he was 5mo old. HOWEVER, he also rang the bell so he could go outside to eat bugs, or check out the neighbor's house if he heard a car pull into their driveway, or if he just wanted to go outside to lay in the sunshine. It didn't matter if we kept him on his leash every time to direct him or not, and it didn't matter how many times we said "Go Potty!" while standing over him - he just kept using the bell for ALL things outside.
Eventually, we put up a little bit of fencing right outside that door so that when he rang the bell, we could just crack the door open and he could run outside and do whatever he wanted, and then he'd scratch on the door to be let back in. That was still a pain, but at least we weren't bending over to leash him up every five minutes.
Then finally, just about a month ago, our landlord was kind enough to install a doggie door for us. And that has made all the difference in the world. Now he goes in and out when he pleases, stays within his small fenced area (which is right by our living room windows, so we can always see him), and we still have no indoor accidents. It's AWESOME.
However, your pup really is still too young to trust with potty training... Just keep making sure that bell ringing = going outside, and nothing else. Treat for potties, no treat for going outside but not pottying. Also, take him on lots of long walks so he's tired and will sleep often. If he's asleep, he's not ringing the bell
or peeing inside.
Good luck,
Lauren & Nikko