The WHOIS/RNDS link used in Special:CheckUser for IPs in 'Get users' and 'Get IP Addresses' is a broken link. The new format is https://www.robtex.com/ip-lookup/<IP> instead of https://www.robtex.com/whois/<IP>.html
The tool does not seem broken, so unless objections for the continued use of this tool are raised, I think the link should just be fixed.
Alternatively the tool https://whois-referral.toolforge.org/ could be used which is a community run tool without ads.