If you don't know, pls tell them you don't know. Otherwise, you have to take full responsibility if you promise to take over the job. Don't forget, your role and responsibility is service KPI. You have failed since day 1 till now! I want you 100% to focus on how to improve the service KPI.
FAE testing is totally different from service and you must be technically strong in order to support this role. I don't know what you have promised but I can't find any difficulty to tell them if you don't know how to do it! If just help with other's assistance, it should be fine but I don't see you can manage to take this task with full responsibility! You don't even discuss with me when you make the decision and how you want me to support you?