That is a cheap radio with very few features. Usually when a servo locks like that it is because the travel is set too far and the servo horn is too short. A longer servo horn with the travel turned down will fix the problem. Only thing is, that radio doesn't have travel adjustment. It has a steering dual rate knob, which will adjust the travel, but it will also adjust the dual rate. You probably bumped the dual rate knob so now the servo turns too far. It's surprising the number of people that use cheap electronics and wonder why they have weird problems. You get what you pay for.