This work presents the construction, testing, and analyses work of the 835-MHz high-temperature superconducting REBCO insert magnet (H835), a critical component of the ongoing MIT 1.3-GHz HTS/LTS NMR Magnet project (1.3G). H835 consists of 40 double-pancake coils operating in a solid nitrogen environment at temperatures ranging from 4 to 17 K. It is designed to generate a central magnetic field of 19.6 T within a 79.4-mm clear bore that will house a 54-mm standard warm bore in the future 1.3-GHz NMR system. Building on lessons learned from our previous 18.8-T REBCO insert magnet (H800), which experienced quenching in 2018 that resulted in permanent damage, several improvements have been implemented in this new H835 design and construction. We have charged H835 to the rated current of 230 A and maintained its central field of 19.43 T for over 15 hours without any issues before ramping down the current. The 0.17-T (0.87%) error field between measurement and design was due to screening current effect. H835 will be combined with the low-temperature superconducting 500-MHz background magnet (L500) to complete 1.3G. We believe that this solid-nitrogen-cooled H835 can provide valuable insights for developing high-field liquid-helium-free REBCO magnets.