Hi, friend! Well, surely you will not be able to correct your problem with the change of oil, filter or anything that has to do with the electronics of the box. The safest thing is that you have to lower it and repair it completely so that the expense is worth it. And I think the problem is in a league or piston (of transmission, of course), in the case of your fault is the reverse clutch. Or it is also possible that the failure is the reverse band. Anyway you will have to lower the transmission. Greetings and oh warn us what happened. It goes.