Hello KNHNEK,
Although everything is working OK on your partner's iPhone, each connection between iPhone and the network is individual so it may be best to delete the VPN and see what happens without it. That will rule out issues there.
Since this happens specifically with cellular data, we'd also recommend checking your cellular data settings for any specific apps that are having issues. The following is from View or change cellular data settings on iPhone - Apple Support.
"Set cellular data use for apps and services
Go to Settings > Cellular, then turn Cellular Data on or off for any app (such as Maps) or service (such as Wi-Fi Assist) that can use cellular data.
If a setting is off, iPhone uses only Wi-Fi for that service.
Note: Wi-Fi Assist is on by default. If Wi-Fi connectivity is poor, Wi-Fi Assist automatically switches to cellular data to boost the signal. Because you stay connected to the internet over cellular when you have a poor Wi-Fi connection, you might use more cellular data, which may incur additional charges depending on your data plan. See the Apple Support article About Wi-Fi Assist."
Another tip from that same article is to make sure that Low Data Mode is not enabled since that can impact your cellular data usage. Check out the following.
"Choose cellular data options for data usage, performance, battery life, and more
To turn Cellular Data on or off, go to Settings > Cellular.
To set options when Cellular Data is on, go to Settings > Cellular > Cellular Data Options, then do any of the following:
- Reduce cellular usage: Turn on Low Data Mode, or tap Data Mode, then choose Low Data Mode. This mode pauses automatic updates and background tasks when iPhone isn’t connected to Wi-Fi."
All the best.
