Introduce or enhance methods of work

Assign dedicated Agile coach to introduce a new framework, or coach teams operating effectively in the existing environment 
In order to support product scaling and delivery extension consider Scaled Agile frameworks like Nexus, SoS, LeSS, SAFe, Spotify, etc.
In case of parallel development of same product by several teams/ streams preserve cadence and synchronization, i.e. align iteration plans and goals, resolve dependencies via cross-collaboration.
Think of switching to another software development methodology or/and framework in case of any changes in complexity, duration, environment, constraints variability, stakeholders availability, etc
Revisit length of iterations to make them shorter for reliable planning and less risks of changes during their execution
When following Scrum, make sure to have all roles, artifacts and events in place as per the Scrum Guide. Ensure that teams understand and follow Scrum pillars and values.
When following Scrum, make sure to have proper time boxing managed for the events, i.e. Sprint Planning - 2-4 hours 
Sprint Review - 1 hour 
Daily Standup - 15 minutes 
Sprint Retrospective - 1-1.5 hours 
Backlog refinement - 1 hour 
If Agile is incorporated into methods of work, check on Agile values and principles to be pursued by the teams and stakeholders.

Useful materials

https://www.scaledagileframework.com

https://scrumguides.org/scrum-guide.html

Pragmatic Agile for a Fixed-Price project