Closed CMS systems and no access to tag manager

I manage Google Ads for a few clients where I’ve built or helped shape their entire digital setup, and in those cases everything runs smoothly.

The challenge comes when working with leads whose websites were built by large agencies with closed systems. I often can’t get access to the site or Google Tag Manager, which makes proper tracking and landing page optimization difficult. It feels like I’m being held back from doing the job properly.

How do you typically handle this situation? Do you build your own landing page on a separate domain? Rely mainly on call ads? Or is there another approach you’ve found effective?

Curious to hear how others solve this bottleneck.