Sorry to hijack this thread, but I had the same issue in JB earlier today.
I entered JB at around 4pm and data worked fine until about 7pm. I could not access mobile data at all, although network shows My Celcom 4G. This is on Xiaomi Redmi Note 4. Tried rebooting phone, reconfiguring APN, on/off airplane mode several times and changing phones but the issue is not resolved. I had to wait until I reached back Singapore (at 9.30pm) until I got mobile data access on the StarHub network.
I remember encountering a similar issue on 18th November 2017 as well, around 12pm at JB CIQ, though this was somehow resolved by itself, so I thought it was a one-off incident back then.
I have used this prepaid SIM card in other countries like Korea and Thailand without any issues at all on the same phone, just that somehow there is an issue with Malaysia. Would appreciate if StarHub could look into this issue.
I cannot test this as I am not in Malaysia today. Thus, it would be great if this issue is escalated to the technical support team who may be able to check if there were any issues yesterday and fix it accordingly.