  We study single crystals of the magnetic superconductor EuRbFe$_4$As$_4$ by
magnetization, electron spin resonance (ESR), angle-resolved photoemission
spectroscopy (ARPES) and electrical resistance in pulsed magnetic fields up to
630 kOe. The superconducting state below 36.5 K is almost isotropic and only
weakly affected by the development of Eu$^{2+}$ magnetic order at 15 K. On the
other hand, for the external magnetic field applied along the c-axis the
temperature dependence of the ESR linewidth reveals a
Berezinskii-Kosterlitz-Thouless topological transition below 15 K. This
indicates that Eu$^{2+}$-planes are a good realization of a two-dimensional
XY-magnet, which reflects the decoupling of the Eu$^{2+}$ magnetic moments from
superconducting FeAs-layers.
