Short answer
Slow internet can happen because of local network conditions, coverage, congestion, indoor/underground locations, or roaming partner limitations.
Common reasons
Weak local signal.
Busy network area.
Indoor, underground, or remote location.
Temporary local network issue.
Switching between local networks.
What to check
Make sure the eSIM is selected for Mobile Data.
Make sure Data Roaming is ON for the eSIM.
Check whether you are in a supported country/region.
If the connection is still unusable, contact support with your current country/city and screenshots of your eSIM settings.