UPC only working correctly on VPN

I first noticed this behaviour when in India, the internet was far more reliable when using a VPN.

I put this down to their network trying to cache files for performance reasons and/ or record all traffic, but since they didn't have sufficient hardware investment, the network was unstable.

After getting UPC, I've noticed a lot of issues especially with google services, but in general, a lot of dropped connectivity on many sites for short and long periods.

Using a VPN results in full internet service restored (despite UPC's [ludicrous?] T&C's).

Anyone surmise if this is bad network configuration on UPC's part or something more sinister or both?

Posting here first because the act of taking the potentially long and frustrating time out of my life to wrangle a (unsatisfactory?) solution or explanation from a large telecoms provider is something I can and will put off.