It is highly improbable that you get your Lunala back. Based on the circumstances of your predicament I am going to assume that you did/do a lot of Wonder Trading. In SMUSUM, your recent passerby list in Festival Plaza only saves the last 20 people you have "encountered," so you do not have a massive list of people you have traded with. If you remember who you traded it to, and they still show up on your passerby list, it is possible that you can get your Lunala back, however it is still unlikely, as you traded it ~2 years ago. The person with your Lunala could very easily not be active or have traded it away himself.
There is another way to acquire a Lunala in USUM without trading, however, it requires you to have a Lunala in your party to get it. If you have any Lunala in your party, and go to the Altar of the Moone, when interacting with the Ultra Wormhole, there will be option to go to 'the reverse world.' If you go to the reverse world, fly to the now-changed Lake of the Moone on Ula' Ula Island. Progress through the building until you reach the altar. At this point, if you have a Lunala in your party, a cutscene will play out, and you will receive a Cosmog. Cosmog evolves into Cosmoem at Level 43, and Cosmoem evolves into Lunala in Moon or Ultra Moon at Level 53. Now, this method is only beneficial if the Lunala you are aiming for is one with your OT. Also, if you have an Ultra Sun game as well, you can get the Cosmog in Ultra Sun, trade it to Ultra Moon, and when you evolve it, it will be Lunala instead of Solgaleo.
It also should be pointed out that because the trader sent you a Shiny Celebi, it's probably likely they are a hacker. You should check the OT on the Celebi, and if it has anything that looks like a web address, (.com, .org, .GTS, .TV, etc) it is a fake Pokemon. It is usually completely legitimate, however not real.
