You can use Mohr-Coulomb (or Drucker-Prager) material model for the soil but you will need more data. Fortunately it shouldn’t be too hard to find it in the literature since there are several articles about such analyses. For example check „Numerical Analysis of Geocell Protective Slope Stability„ by G. Wang et al.
When it comes to the geocell mattress itsefl, it can be modeled as linear elastic and meshed with membrane elements. Check also this article for reference: „Efficiency of Geocell Reinforcement for Using in Expanded Polystyrene Embankments via Numerical Analysis” by S.N.M. Tafreshi et al.
