In this paper, a novel demapper for two-dimensional non-uniform constellations (2D-NUC) is proposed, exploiting the characteristics of these constellations. It represents the combination of two underlying demapping techniques targeting ATSC 3.0 compliant OFDM transceiver. On the one hand, for low code rates, we define a metric to perform condensed demapping. On the other hand, for high code rates, adaptive sub-region demapping is proposed. In this work, a combination of both demapping methods is designed showing comparable performance to the classical Max-log demapper since the gap does not exceed 0.1 dB for all code rates of the ATSC 3.0 standard. Higher complexity reduction, from 79.2% to 95.4%, than state of art 2D-NUC demappers is obtained for 2D-256NUCs. These results are validated for ideal and non-ideal channel state information (CSI) over additive white noise Gaussian (AWGN) and Rayleigh independently and identically distributed (iid) channels. Results are extended to 2D-1kNUCs and 2D-4kNUCs showing demapping complexity reduction from 96% to 99.7% with a negligible impact on performance.