Look up a product called Thrift. You can get it on Amazon. It's a type of flake drain cleaner, comes in a bottle. Basically run your shower on super hot for a few minutes to heat up the pipe, then pour a bunch of this stuff down, and a bit more hot water. Leave it alone for a few minutes, then flush with hot water. Unlike Drano, this stuff actually works. Originally I found it on a plumbing subreddit. I've done it to my shower a few times and it seems to have a longer lasting effect every time, like it's actually cleaning out the pipe.