This router has only 16 MB of storage size. I have RBwAP2nD and RB760iGS as well. On RBwAP2nD I had upgrade problems just because of storage size. And I solved this by getting rid of unwanted packages. Go to System > Packages and uninstall packages you don’t use. Now on both routers I have only these packages left -
But I think this won’t fix your issue with PPPoE clients. Storage size should not affect authentication problems.