A search for decays of neutral long-lived particles (LLPs) is presented using 139/fb of proton-proton collisions data collected by the ATLAS detector at the LHC in 2015--2018 at a center-of-mass energy of 13 TeV. Dedicated techniques are developed in this search for the reconstruction of displaced jets produced by LLPs decaying hadronically in the ATLAS hadronic calorimeter. Two search regions are defined for different LLP kinematic regimes. The observed numbers of events are consistent with the expected background and limits for several benchmark signals are determined. For a scalar boson with a mass of 125 GeV, branching ratios above 10% are excluded at 95% confidence level for LLP proper decay lengths in the range between 2 cm and 10 m depending on the model. Upper limits are also set on the cross-section times branching ratio for heavy scalars with a mass of 60 GeV and for masses between 200 GeV and 1 TeV.