Ahh, thank you! That makes sense. I did not know that the USSD code transmission relied on specific frequency bands. I looked at frequencycheck.com, and what I see is that in my country mobile networks and carriers use 2 GSM bands, 2 UMTS bands, and 4 LTE bands.
So... A USSD code would have to be transmitted one of the GSM bands? And the mobile router does not support these bands natively. Very little I can do if this is the case, I suppose.
I see... I understand. Thank you!
It is a bit unfortunate because my mobile carrier only allows me to top-up via USSD or via a phone call, not via SMS. So I won't be able to top up without removing the sim card and placing it into a phone. It is a minor inconvenience, but an inconvenience nonetheless ๐