Issue with IL2CPP where Default Constructor is possibly stripped from assembly

Apologies if this is not the right place to ask about this error. I've been at this for 4-5 days.

Specs: protobuf-net 3.0.73 integrated into a Unity 2019.4.6f1 project that targets .NET Standard 2.0 when building for Xbox console with IL2CPP set to Low Stripping. Exact log: "MissingMthodException: Default constructor not found for type ProtoBuf.Internal.PrimaryTypeProvider"

The project built in Editor works exactly as intended and that's why I think it's the IL2CPP stripping. Error is reported when attempting to assign a new data field of ProtoTest. Might be important to note that the ProtoTest class is a C# Library DLL coded and programmed outside of the Unity environment, as required per ProtoBuf Custom serialization. (To my knowledge). I then built it and ported it into Unity Plugins folder along with the matching Serializer.

 ProtoTest saveFile = new ProtoTest(); // This is the call made where the code will fail

 ----- // Decorated class
 [ProtoContract()]
 public class ProtoTest
 {
     [ProtoMember(1)]
     public string testString;
     [ProtoMember(2)]
     public int testint;
     [ProtoMember(3)]
     public int testBool;
     [ProtoMember(4)]
     public int[] testFixedIntArray;
     [ProtoMember(5)]
     public List<int> testList = new List<int>();
     [ProtoMember(6)]
     public Dictionary<int, string> testDictionary = new Dictionary<int, string>();

     // This is the constructor that is being stripped (to my knowledge)
     public ProtoTest()  { }
 }

I suspect that some off the protobuf files are being stripped during the build, I don't have a lot of experience with handling this. However I attempted to omit the protobuf namespace from stripping by writing a Linker referencing the namespaces used to create all the protobuf data I know of.

Might be important to note that the ProtoTest class is a C# Library DLL coded and programmed outside of the Unity environment, as required per ProtoBuf Custom serialization. (To my knowledge).

Attempt 1

 <linker>
   <assembly fullname="ProtoBuf">
     <namespace fullname="ProtoBuf.Internal.PrimaryTypeProvider" preserve="all"/>
   </assembly>
 </linker>

Attempt 2

 <linker>
   <assembly fullname="ProtoSave" preserve="all"/>
   <assembly fullname="ProtoBuf" preserve="all"/>
   <assembly fullname="ProtoBuf.Internal" preserve="all"/>
   <assembly fullname="ProtoBuf.Meta" preserve="all"/>
 </linker>

Attempt 3

 <linker>
   <namespace fullname="ProtoSave" preserve="all"/>
   <namespace fullname="ProtoBuf" preserve="all"/>
   <namespace fullname="ProtoBuf.Internal" preserve="all"/>
   <namespace fullname="ProtoBuf.Meta" preserve="all"/>
   <namespace fullname="ProtoBuf.Serializers" preserve="all"/>
   <namespace fullname="DataSerializer" preserve="all"/>
 </linker>