A “small number” of Plusnet mobile network users have been unable to access 3G and 4G data services for almost a week.
The BT-owned company said the problem only affected those who had received new Plusnet Sim cards within the past seven days.
After installing them, users have been able to access voice and text services but not mobile broadband.
Plusnet said it was working to resolve the problem “as quickly as possible”.
However, it would not comment on exactly how many customers had been affected.
“We’re aware a small number of customers who… have been affected by a configuration issue, which means they can’t access data services,” a representative told the BBC.
Poor customer service
Users reacted angrily on the company’s community forum, with some saying that replacement Sims subsequently sent out had also not worked.
Others complained that Plusnet’s customer service had been poor, including GMcNeill, who posted on Tuesday: “I’m new to Plusnet and have today had my mobile number ported across successfully (within two working days [as] stated), but I have no mobile data.
“I’m not seeing any announcements to customers about this – would have saved me a lot of time checking my APN [access point name] settings if Plusnet had been more proactive in making us aware of this known issue.”
On Sunday, user BarN posted: “This is now the third day without mobile data.
“My patience is wearing thin, and I’m sure many others feel the same.”
Plusnet’s network was launched in November and piggybacks off the BT-owned EE network.
All three companies were recently criticised for their fixed broadband services, having been ranked the most complained-about ISPs in the third quarter of 2016, according to regulator Ofcom.