Maugeter's world has no access to the outer planes. Demons, devils, celestials, slaad and all the other countless varieties of outsider have never seen it. The words demon and devil have contextual meaning - a Maugetan would be referring to an evil spirit of some kind - but they will never have seen or heard of an actual Tanar'ri or Baatezu.
It does have inner planar access, so Elemental creatures have full access. In theory, an outsider could reach Maugeter through the inner planes, but apparently so far none have done so. The land's multiracial culture almost encourages the creation of half-elementals, and while they still aren't common, it isn't rare to see a being with a slight tinge of red or blue to their skin.
Access to the astral, shadow and ethereal planes is possible, but those planes seem restricted in some way - you cannot use them to travel beyond Maugeter's world. Whether they are the actual planes in question or simply minor, local copies of those planes is an unanswered question for sages - and since they don't know anything is amiss, few are looking for an answer.