The cyclostationary noise in low-voltage narrowband powerline communications (LV NB-PLC) severely degrades the system's reliability. In this paper, we adopt single-input multi-output (SIMO) transmission to enhance the reliability of NB-PLC. In addition to SIMO diversity and power gains, another SIMO advantage in the LV NB-PLC is the high cyclostationary noise correlation observed over the different receive phases which can significantly help in noise mitigation. Considering the SIMO receiver structure, we propose a temporal-region-based cyclostationary noise mitigation technique that minimizes the mean-squared error in estimating the frequency- domain information signal. The proposed technique exploits the cyclostationarity of the noise to estimate its power spectral density as well as the cross-correlation per frequency subchannel over multiple stationary noise temporal regions. Our proposed SIMO NB-PLC noise mitigation technique is shown via simulation results conducted using field noise measurements to achieve considerable performance gains over single-input single-output noise mitigation techniques and over the conventional SIMO maximal-ratio-combiner designed assuming stationary noise.