I lived in US for around 10 years and moved to Korea in 2016. Worked at a rural hagwon for 2 years teaching elementary and middle school students. I started my military service on January 2019 and I finished on October this year. My Korean is still at conversational level so I'm having trouble looking for work that's not ESL related. I do not have an undergraduate degree.

Do you have a US passport?  Dual citizen?  If so, get a US university degree or get a Canadian degree from a small town university if your student loans transfer there.  (Your exchange rate with Canadian dollar is good.  So, it may be cheap or a State college may be cheaper instead.  Then you can teach here as a native speaker or any other country for that matter and make better money than as  Korean.)