In Belgium Neanderthals were observed in only 2 time periods :
first the Eemian, which was a warm period, even warmer than today
second around 40 ka, just before the Neanderthals became extinct and were replaced by AMH
I think is says enough about the capability of Neanderthals to survive in this area
the Neanderthals were unable to live in northeren latitudes during glacial periods, except in areas like Moravia and the Altai were there was an abundance of large animals
I suspect that even in these areas they didn't survive the glacial maxima
after each glacial maximum the north was recolonised by Neanderthals from a refugium along the Mediterranean or in the Levant, hence their small genetic variation
the Neanderthals in the Altai from before the Eemian were replaced by other Neanderthals during the Eemian, from a different genetic clade
the oldest Neanderthals had mtDNA DN2
https://www.yfull.com/mtree/DN/
while the younger ones carried mtDNA NA
https://www.yfull.com/mtree/