Bad data speeds, connects at 4G then goes to EDGE

My parents have had the same problem for about a month with the Cell C SIM card that they recently topped up with data. The issue appears when using in both their 3G USB ****le and phone. When the phone is rebooted, or USB ****le is plugged in they will initially get good speeds, 3G or 4G, however after a couple of minutes it will fall back to EDGE connection and be unusable. They have had the issue in Rosebank, Mowbray and Kenilworth in Cape Town.

They took it to the Cell C shop in Kenilworth Centre where someone checked that the SIM card was fine and getting 4G, but he did also "reinstall the network settings" just in case. I don't think the person in the shop tested it for long enough as the issue only seems to appear after a couple of minutes. My parents have not had previous issues with the network in their area and have been using the same SIM card for a couple of years. Can you please assist.