The way roaming works is that traffic will have to go back to your home network first before going out to the internet. If you’re camped to say PartnerA and no data moves, it could means that the tunnel between PartnerA and Eight is congested, offline, or otherwise not available. In this case, you can try PartnerB, if available.
The other thing to take note is that Eight offers 4G plans. Roaming partners may let you camped onto their 5G network for whatever reason. Later, it may then decide to kick you back to 4G. The transition between 4G and 5G radios and network can be problematic at times. To avoid such issues, just tell your phone to use only 4G network. If you’re already stuck, then set to airplane mode, wait 10 seconds, and toggle it off.
At the worst, buy a local SIM card.