Locked phone and eSim?

I have a Samsung s23 Ultra I bought from Samsung a few year ago (long time Tmobile/Sprint customer).

I did not know but is apparently unlocked even though I was on an installment plan.   I travel often and install an eSim and now have 1 from Thailand that I do not want to lose as my Thai number is tied to my Thai (online) bank account also.

I want to upgrade to the Samsung s25 Ultra but Tmobile tells me I cannot transfer and use my Thai eSim since the S25 will be on an installment plan (24 months).  

I do not understand why not as I will still use/keep my Tmobile Sim.    Also why do they have eSim anyway?

I see there is a feature on my S23 to temporarily unlock the phone, I guess to use the eSim but it does not say how long.     When I Google is implied 30 days but I am away sometimes more than that.   Could I just unlock it again (after the 30 days expire)? Is the temporaty Unlock request fast?   Do I need to do it from the USA or can I do it overseas (my plan wth Tmobile is free overseas).

Thanks!