I'm not Requestbin's creator so I don't know. A simple nslookup or curl does work for me, with my system's DNS servers set to Cloudflare (1.1.1.1) or Google (8.8.8.8)
It looks like Vodafone (I assume this is your ISP) DNS servers aren't properly resolving the name for some reason. You could try bypassing it with dig, and directly ask a different DNS server to resolve it:
dig @1.1.1.1 A whatever.a54c4d391bad1b48ebc3.d.requestbin.net
It looks like Vodafone (I assume this is your ISP) DNS servers aren't properly resolving the name for some reason. You could try bypassing it with dig, and directly ask a different DNS server to resolve it: