New sodium-based battery concepts require solid electrolytes as ion conducting separators. Besides NaSICON and b-Al2O3 in the Na2O-R2O3-SiO2 system (R = rare earth), a rarely noticed glass-ceramic solid electrolyte with the composition Na5RSi4O12 (N5-type) exists. The present study addresses the investigation of the ionic conductivity of Na5RSi4O12 solid electrolytes sintered from pre-crystallized glass-ceramic powders. The sintering behavior (optical dilatometry), the microstructure (SEM/EDX), and phase composition (XRD), as well as electrochemical properties (impedance spectroscopy), were investigated. To evaluate the effect of the ionic radii, Y, Sm and Gd rare elements were chosen. All compositions were successfully synthesized to fully densified compacts having the corresponding conducting N5-type phase as the main component. The densification behavior was in agreement with the melting point, which decreased with increasing ionic radii and specific cell volume. Alternatively, the ionic conductivities of N5-phases decreased from Y to Gd and Sm containing samples. The highest ionic conductivity of 1.82·10−3 S cm−1 at 20 °C was obtained for Na5YSi4O12 composition. The impact of grain boundaries and bulk conductivity on measured values is discussed. A powder-based synthesis method of this glass-ceramic solid electrolyte using different rare earth elements opens possibilities for optimizing ionic conductivity and scalable technological processing by tape casting.