How can I play League of Legends at Korean server?

I'm from SEA and have played on all major Asian regions up to diamond/challenger.

You can create multiple accounts with 1 korean SSN, so if you have any korean friends, they can create an account for you.

Otherwise, you can try befriending some koreans on discord / other forums using monetary gifts.

If you're just there to test yourself, you can get a temporary account from any of the "korean account selling sites" that will likely get banned within a few months, enough time to squeeze a few hundred games in and get diamond+. You can find these sites easily via google. (A Chinese Kayn player managed to get rank1 earlier this season with such an account)

Since you're likely from Bangladesh, have you considered Ionia instead? It should be closer to you since it's located in southern China. You can get access by creating a QQ account and buying a VPN such as Xunyou.

You can also play on Garena TH or SG, both of which offers competitive games up to ~plat+