You can contact Upaycard and report complaint about illegal things on Legal Inquires mail address :

legal@upaycard.com

https://www.upaycard.com/contact


Mastercard you can contact on UK address (because Upaycard is under FCA - British regulations)

United Kingdom
10 Upper Bank Street
Canary Wharf
London, United Kingdom E14 5NP  
Telephone: 44.2075.575.000


Also you can fill official complaint form on Mastercard site:

https://www.mastercard.us/en-us/ask-mastercard-webform.html


Or on Twitter:

https://twitter.com/askmastercard?lang=en