Hello! The frostwurm tunnels are very high in the mountains near Hayle so it would not be possible at this time to get there due to Hayle's closure.

However, similar icy tunnels can be found closer to northern areas of Nott if you would like to use the setting for similar purposes. They may not have the same wurms, but other creatures can be found. Thancred would be able to travel there and back by horse as he wishes, especially as he's a resident of Nott now.