I have just purchased an Archer MR400 MiFi router and need to use it to allow a SumUp Solo card reader to be used in an area with no WiFi signal. 
The router seems to be working well when connected to my iPad.  I can select the router from the WiFi connection screen of the SumUp card reader but when I try to make a credit card transaction the card reader fails to process the payment, and gives an error message: 'Error: could not connect to the server'.  (Note:  if I connect the card reader to my home WiFi there is no problem and the payment is processed normally.) 

I think there may be a lack of security in the router set up that is causing this problem.  Can I improve the security? Perhaps this will overcome this problem?