The game is not functioning as it should - I have completed all of the Lumbridge achievements, but it won't let me get my Explorer ring 4. I have submitted a bug report several days ago, but have had no response. Where do I get this fixed?
I'm afraid Jagex doesn't respond to individual bug reports. If they do, it's usually in extreme cases, such as the loss of items through death.
It looks like the handing out of rewards was patched with the last update. Make sure you're talking to the correct individual following all chat options and that you have actually completed all necessary achievements. I've not seen any other threads or post claiming the issue has continued, so maybe it's something on your end.
Nevertheless, if the problem does persist, submit a further report.