You already know about default, so use it for your title link:
<a href="{{ .Params.link | default .Permalink }}">{{ .Title }}</a>
In that instance, link is the name of my front matter variable. If yours is linkurl, use that instead.
And if you want any more assistance, please read Requesting Help. There is only so much we can do without a repo to look at and reproduce the issue you are having.