The design of microstructures is essential for tailoring the microwave dielectric properties of ceramics, yet the structure–property relationships in tetragonal scheelite-structured ceramics remain insufficiently understood. In this study, first-principles calculations combined with experiments were used to systematically investigate the interrelations among the sintering behavior, crystal structure, electrical characteristics, bond characteristics, and dielectric performance of NaSrLnMo₃O₁₂ (Ln = Ce, Pr, Eu, Y, and Yb) ceramics. All the compositions crystallized into a tetragonal scheelite structure (space group I4₁/a) and exhibited favorable dielectric properties with optimal sintering temperatures of 775–925 °C, ε_r = 9.8–10.34, Q×f = 30,487–69,445 GHz, and τ_f = −20.55–(−44.24) ppm/°C. The increase in ε_r originated mainly from the increased ionicity of the Na/Sr/Ln–O bonds, whereas the increase in Q×f was attributed to the increased lattice energy of the Mo–O bonds, increased bond valence, and reduced ionic/electricity disorder. The negative shift in τ_f was primarily linked to the increased linear thermal expansion coefficient α_L of the Na/Sr/Ln–O bonds. Furthermore, the electrical characteristics and relaxation mechanisms were examined, and the dielectric response in the terahertz range was confirmed. Finally, NaSrCeMo₃O₁₂ was employed to design and fabricate two antenna devices, verifying its potential for high-frequency communication. This work provides a systematic understanding of the role of Ln in optimizing the dielectric properties of tetragonal scheelite ceramics and clarifies the microscopic mechanisms underlying their performance.