Would suggest you get a degree in nutrition, probably another masters degree. To get info about becoming an RD go to the Academy of Nutrition and Dietetics site. You may have to take a lot of prerequisite courses to meet the Academy’s educational requirements that were not met by your exercise physiology degree. Then after graduation, you have to complete a 9-month internship or professional experience combined with a master’s degree. There are CUP programs that combine an undergraduate degree with the internship and could take 3 years if you have all the general education prerequisites. So that is why I suggested the master’s degree as your best option.
FYI, some states now have licensure that restricts who can provide nutrition advice within a state. Check on licensure with your state dietetic association which is probably located in the capital of the state in which you live.