It is unreasonable to expect that something which occurred spontaneously and personally, to overnight be done in a new way, a new place, without fail. Children don't feel the urge to urinate until their bladder is full. If toiletting becomes a battle of wills, you both loose. Regression may be caused by stress, tension, change in routine or life events.
Toddlers can handle only one change at a time.
It usually means child is not yet ready. Postpone for awhile. By starting later, the total time of training may be less.