I remember they were having some problems in February when I tried to topup. But, that was supposed to be fixed in April when they move to a different merchant. Looks like it's still a problem. Did you try it from their website and this other hosting site?
http://longdistancesim.host4tele.com/hosting/globalsim/
Here is my posting and a response from Travis at freeGlobalsim regarding topup problems.
http://www.prepaidgsm.net/forum/show...=freeglobalsim
Phones: Xiaomi Mi Mix 2, Samsung Galaxy A50, ASUS zenfone 3,
Sim cards: AT&T (Contract), 3 UK, Piranha Mobile