We don't call "웨이터" to a server works in normal restaurants. 웨이터 is used for a waiter who works in (adult) entertainment spots or night clubs or a fancy restaurant. Normally 웨이터 wears formal dress like a white shirt and tie. To order, you can call them "여기요 or 저기요 or 웨이터" We use 식당 종업원 for a server who works in normal restaurants. You can use 식당 종업원 in writing or in a talk to refer to others. By the way, we usually don't call "식당 종업원" to a waiter in person. To order, we call them "여기요 or 저기요 (means here)". When we are confused if someone is 식당 종업원 or not in the restaurant, we ask them "여기 직원이세요? Are you a staff member here?" not 식당 종업원 or 종업원. It's a bit impolite. 직원 sounds better when you refer to them in person.