organic chemistry - How exactly does the reaction between alkyl halides and silver nitrite occur?

$$\ce{R-X + AgNO_2 -> RNO_2}$$ Now depending on the $\ce{R}$ and solvent the reaction can occur via $\mathrm{S_N1}$ or $\mathrm{S_N2}$.
When the reaction proceeds via $\mathrm{S_N1}$ the product formed is $\ce{R-O-N=O}$ whereas for $\mathrm{S_N2}$ it is $\ce{R-NO_2}$.
Why is this so? According to me as the $\ce{Ag-O}$ bond is covalent the lone pairs on nitrogen must attack in both the cases forming the product $\ce{R-NO_2}$.