Since you packedd the dependent dll with your NuGet package. When the NuGet package is consumed by your project, the dependent dll will exist in ..\packages\{yourNugetPackage}\lib\{targetFramework}\dependent.dll
.
So you can manually add a Reference in your project to this dependent dll. For below example.
<ItemGroup>
<Reference Include="DependentDll">
<HintPath>..\packages\yourNugetPackage.1.0.0\lib\netcoreapp2.0\dependent.dll</HintPath>
</Reference>
</ItemGroup>
Another possible workaround is that you can try creating another Nuget package for this dependent dll and publish it to the feed in azure devops.
Then add dependency to this package in your original Nuget package project and republish to feed in Azure Devops. So that the dependent dll can be managed with your original Nuget Package. See below simple example.
<package xmlns="http://schemas.microsoft.com/packaging/2010/07/nuspec.xsd">
<metadata>
<id>sample</id>
<version>1.0.0</version>
<authors>Microsoft</authors>
<dependencies>
<dependency id="dependentDllPackage" version="1.0.0" />
</dependencies>
</metadata>
</package>