Why it is showing NODATA??
Judging from the filenames in the screen provided, you are not using ALOS DEM but SRTM data that accompany ASF ALOS RTC products. Despite a widespread misconception, those are not ALOS DEM but up-sampled SRTM elevation tiles, 30m original resolution, used for terrain correction in ALOS radar images by the ASF. 12.5m resolution is only for the product raster file not original data. https://media.asf.alaska.edu/uploads/RTC/rtc_product_guide_v1.2.pdf
So you should use either the original SRTM1 data or the ALOS DEM tiles, that are already available and free.