Cell C Technical help doesn't deliver

This is re my data line # ***. I use a ZTE MF283+router supp**** by Cell C. There is a problem in that in the router dashboard I can't open/read SMS messages received - all of which come only from Cell C. Pop-up message states "SMS capacity is full. Please delete some SMS". When I click on the SMS icon or on the SMS menu option the dashboard indicates processing but no result delivers, ie: SMS file doesn't open. So, catch 22: have to delete some SMSs but can’t open the file to do so! Emailed Customer Support (Case ) to ask for help, they told me to phone Technical ***. Did so last week - spoke with Gugulethu (?). Could not answer - didn't understand the issue (I think) and had to put me on hold while she consulted . Still couldn't provide a solution for me to resolve on screen/in the dashboard. Her answer was to remove SIM and try using it in another device! This is manufacturer supp**** hardware and (I think) manufacturer software but it’s a device supp**** by Cell C. Surely Cell C technical agents are sufficiently trained and capable re hardware/software that the company supplies to its customers to be able to provide solutions?