Public mmWave radar datasets are collected under heterogeneous acquisition configurations and processing protocols, leading to systematic geometric and signal-statistical shifts across datasets. Even when the underlying hardware is similar, differences in grid resolution, field-of-view, measurable range/velocity, and the availability of a calibrated Doppler/velocity axis make naive multi-dataset aggregation unreliable and can bias models toward dataset-specific artifacts. We propose MetaARM, a metadata-conditioned self-supervised pre-training framework for heterogeneous mmWave radar perception. MetaARM partitions a 3D radar tensor into non-overlapping 3D patches and trains with causal next-patch prediction under a structured serialization order: tokens are first ordered along the sequence axis and then raster-scanned within each Range–Azimuth plane. To explicitly handle configuration shifts, MetaARM injects acquisition metadata through two coupled mechanisms: (i) Absolute Coordinate Encoding (ACE), which converts discrete patch indices into normalized physical coordinates using resolution and FoV metadata to reduce geometric inconsistency; and (ii) FiLM-based conditioning inside Mamba blocks, which calibrates token features before the selective scan according to observable acquisition/configuration metadata. Together, these pathways use the same metadata to link physical radar-token geometry with configuration-aware sequence modeling before autoregressive prediction. Pre-trained on an aggregated corpus of five public radar datasets, MetaARM improves transfer performance on three benchmarks, achieving 64.5 mAP@0.3 on RADDet, 49.5 mIoU on CARRADA (RA), and 85.0 AP on CRUW, improving over matched training-from-scratch, MAE pre-training, joint supervised, and lightweight normalization and metadata-affine calibration baselines under the reported matched settings. Overall, the results suggest that metadata-conditioned autoregressive pre-training is a practical way to learn radar representations that transfer better under cross-dataset acquisition/configuration shifts.