Compactified 2HDM under the Non-SUSY AdS instability conjecture

We investigate how extra-dimensional dynamics influence the Higgs sector phenomenology by compactifying a Two-Higgs-Doublet Model (2HDM) coupled to 4D gravity on a circle $S^1$. The resulting effective potential includes tree-level 2HDM interactions, one-loop Coleman-Weinberg corrections from the Kaluza-Klein towers, and a radion contribution inspired by the Goldberger-Wise mechanism. We derive the full 3D effective action and show that, for the observed Higgs mass $m_h=125$ GeV, the radion potential admits a stable minimum, near-zero vacuum energy. By imposing the non-supersymmetric AdS instability conjecture as a quantum gravity consistency requirement, we obtain a model independent bound on the heavy Higgs sector, finding that the additional scalar states must satisfy $M_H\gtrsim 680$ GeV to avoid an unstable AdS${}_3$ vacuum. Our results demonstrate how Swampland inspired constraints can yield sharp, phenomenologically testable predictions for extended Higgs sectors.