So I tried to block roaming on my sim several times this week but it just refuses to work, everytime I try it, it says "sorry, the service is unavailable, try again later". What do I do to fix the problem?
Are you trying in a router, and if so, which one ?
Just looked in my Huawei B525 settings, under Mobile Network - Internet Connection ; there's a toggle switch to turn Data Roaming on / off so maybe try look for something similar if you're using a different router.
EDIT : I saw afterwards you're trying via *180# , so via USSD - presume you're using the SIM in a smartphone then ?