Rooting questions*sry im new to rooting*

I rooted and used a custom rom(korean rom) for note 5 and used Region Changer to change it to Singapore(im from singapore). Im from singapore and im using a korea based rom,but using service provider from singapore(specifically M1) . Used a region changer to change phone region from korean to sg(not sure whether phone(bought from SG but used a korean rom not sure if it is in korean region so i changed it to sg) because all the incoming calls the numbers are shown in korean format(korean country code format instead of +65). Wondering if it will incur extra cost to bills if i use mobile data since i used a korean rom but have already changed region to singapore using region chnanger from googleplay? Thanks in advance.

Thanks @sinkster! Is it normal that my service provider which is M1 shows up as MobileOne(its old name before they changed it) on the custom rom? Sorry very new to rooting