Taiwan Mobile Phone stuck on 4G+

Hey everyone, I have a Taiwan Mobile simcard that I often switch between my two phones. I'm from the UK where I would normally use 5G mobile data for my mobile internet usage. However my sim card here seems to max out at 4G+. Is this just an issue with my personal plan or is this the norm in TW? Do I need to upgrade my plan in store? Please let me know if you have any insights.