Did you buy it online? I have been using OnePlus from the one and now 7 plus and it works in every country. Even in Thailand and Philippines and Maldives and every country I have been to.
i agree with you that the people at the mobile shops don't know much and especially the OnePlus.
You need to take it to MBK and ask a tech shop there. They will fix it as they sell OnePlus a lot as lots of foreigners buy them there too and are familiar with the OnePlus unlike the mobile shops like True, Dtac or AIS.