Vivaldi on iPhone and iPad supports custom search engines by URL.

1. Open the Vivaldi menu, then Settings > Search Engine > Add Custom Search Engine.
2. Set the name to Queye.
3. Pick a short nickname such as qy.
4. Set the search URL to https://queye.co/search?s=%s.
5. Save it, then choose Queye under Standard Tab if you want it as the default.

Vivaldi on iOS can also add a custom engine directly from a website by long-pressing its search field.