Most robots are not going to climb the walls on their own. If you can get by with just cleaning the floor and brushing the walls by hand, then you might find a lot of unit will do a good job for you. I personally have an Intex auto pool cleaner that does a great job on the floor of my 24ft round agp. I only need to brush the walls maybe once a month if that often.
The Intex robot was cheap, at only $60-$80 price range, you might want to give it a try.
It does have a large fitting that you will have to adapt if you are not putting it into an Intex brand pool. I just "t"ed into my return line and added 2 ball valves to allow me to shut off the flow back into the pool and direct it all into the robot. Worked too good, I had to open the return valve to the jet some because the robot was coming up off the bottom and launching out of the water. Lol
Tim