Purely from Hotlink Postpaid app, as far as i'm concerned. After Maxis revamped their website, a lot of self-serve functions have been removed. So though Maxis postpaid subscribers can still login to access their account information on Maxis website, there isn't a lot of information available and you can also no longer activate or deactivate a lot of value-added services like call forwarding, block caller ID etc.
Don't call IDD and don't toggle on roaming on your phone when you are overseas. Safest is probably to set your phone to plane mode.

Or if you would like to know if someone is calling you or continue to receive SMS when you are overseas, just call 123 and ask Maxis to disable your overseas voicemail service. This way if someone calls you your phone will still ring but as long as you don't answer the call, you won't be charged for it. For SMS, receiving SMS in overseas is always free. The reason some people are billed even they don't answer the call whilst overseas is because the call is re-directed to your voicemail. So the solution is to disable your roaming voicemail service. Do note this also applies to Digi.

Suddenly got 5GB about loyalty rewards valid for a month

Mine already upgraded few days after received the sms.

Yes, received the upgrade few days after SMS.