Please advise on how to set up access to the NAS when the modem has only an IPv6 address

My son has O2 as a provider in Prague. The ADSL router has the provider’s firmware with limited settings. Only an IPv6 address is available; obtaining IPv4 and static addresses is challenging, time-consuming, and expensive. I need SSH access to my NAS from any external IPv4 address. I have access to the Terramaster NAS settings and require SSH specifically for data recovery.

I’ll appreciate any advice.