Robos are especially hyper and prone to aggression. It's really hard to retrain a biting hamster in their short lifespan. I don't even bother. If I want tame hamsters I breed my own because pretty much the only place to get hamsters are the mills selling to petstores and not handling them as babies. Some day I will make it to the national rodent show to buy from breeders when it comes near me again. You can try giving 1 seed or nut chip at a time and make him keep coming back for more but robos are so small it's hard to feed them little pieces that won't take them long to eat without getting your fingers nipped. Putting stuff on a spoon can work with some small animals instead of feeding from your hand. Biting the spoon also isn't very comfortable or satisfying so sometimes it can deter them. Hamsters and gerbils also can enjoy many insects sold for reptiles.
Personally I will only feed a high quality mix. Hamsters and gerbils evolved to forage a mix of food items and pellet often are not formulated with the ingredients they would normally have in a loose diet. Harry or hazel hamster food has always been listed as one of the best and you can add some hay (this is controversial with some) and small amounts of fresh food. Chunks of stem and leaf of common herbs like cilantro, parsley, or mint, cut up cranberries, and carrot peelings work well. The only downside of a good mix is that you have to be careful they eat all of it. It takes practice to know exactly how much your hamster is eating and not make them go hungry or find large stashes when you clean the cage.
