Cloudformation NATGateway replacement: traffic blocked from vpc

I am attempting to swap out a NATGateway instance in my Cloudformation Yaml script with a Nat instance AMI named Fck-Nat in order to reduce the (insane) cost from $40/m to around $4/m.

My current VPC Diagram from the console

The key difference:

 # NatGateway:
 #   Type: AWS::EC2::NatGateway
 #   Properties:
 #     AllocationId: !GetAtt ElasticIp.AllocationId
 #     SubnetId: !Ref PublicSubnet

 VpcPrivateRouteTable:
   Type: AWS::EC2::RouteTable
   Properties:
     VpcId: !Ref StaticIpVpc

 PrivateRoute:
  Type: AWS::EC2::Route
  DependsOn: VpcAttachment
  Properties:
    DestinationCidrBlock: 0.0.0.0/0
    RouteTableId: !Ref VpcPrivateRouteTable
    # NatGatewayId:  #!Ref NatGateway # - OLD Hook <===============
    NetworkInterfaceId: !Ref FckNatInterface # New Hook <===============Replaced with

PrivateSubnetRouteTableAssociation:
Type: AWS::EC2::SubnetRouteTableAssociation
Properties:
  RouteTableId: !Ref VpcPrivateRouteTable
  SubnetId: !Ref PrivateSubnet

'FckNatInterface' is the same as mentioned in the docs Fck-nat deployment (Cloudformation section)

Everything deploys without error, however the lambda function cannot access the ElasticIP or otherwise reach the outside world see image below of the axios response from inside the function:

I have checked the routes, ipv4 allocation, and as far as I can tell, the architecture is fine. I have also re-enabled the NAT-Gateway to ensure no mistakes where made elsewhere (all worked as intended). So the issue is 100% with fck-nat bridge from private to public, but I'm not enough of an expert in network to debug further.

Any advice is welcome. Hopefully this also helps others.

Other info:

• A staticIP is a requirement for my lambda Function due to security
  requirements of LimeLM,LimeLm Forumn link

• Please let me know if additional information required in the
  commnents, I will update as needed

• See Here for a fairly close varient of what my implementation is with NATGateway instead of fck-nat.

• A great general guide explaining VPC Architecture click Here

I would start by checking the Security Groups on the NAT instance and ensuring that the instance has IP forwarding enabled net.ipv4.ip_forward=1.

Also, ensure Source/Destination Check is disabled for the EC2 instance.

If Lambda is accessing resources by their domain name, ensure that "DNS resolution" is enabled in the VPC settings.

If, at some point in the future, you choose to look into alternative solutions to custom-built NAT instances, I would recommend looking into the Aviatrix Secure Egress solution, known to reduce NAT costs and provide FQDN filtering as a bonus without too much manual plumbing.

We have a TCO calculator that can help figure out the difference in expenditure here:
https://aviatrix.com/tco-calculator/

Disclosure: I architect networks for Aviatrix

Finally worked out an answer! :

1)
The Internet gateway has to be connected to the public route table:

PublicSubnetRouteTableAssociation:
  Type: AWS::EC2::SubnetRouteTableAssociation
  Properties:
    RouteTableId:
      !Ref VpcPublicRouteTable
    SubnetId:
      !Ref PublicSubnet

2)
The 'AWS::EC2::Instance' requires a network interface with a 'groupset' that allows all traffic through, rather than a direct 'network interface' connection

FckNatEC2Instance:
Type: AWS::EC2::Instance
Properties:
  Tags:
  - Key: Name
    Value: fckNatInstance
  SourceDestCheck: false
  ImageId: ami-084b3eca5402436bf
  InstanceType: t4g.nano
  NetworkInterfaces:
  # - NetworkInterfaceId: !Ref FckNatInterface OLD
  # NEW:
  - DeleteOnTermination: true
    Description: fckNatEC2Instance network interface
    SubnetId: !Ref PublicSubnet
    GroupSet: 
      - !Ref SGAllTrafficFromVPC 
    AssociatePublicIpAddress: "true"
    DeviceIndex: "0"

Add:

SGAllTrafficFromVPC:
Type: 'AWS::EC2::SecurityGroup'
Properties:
  GroupName: SGAllTrafficFromVPC
  GroupDescription: VPN Traffic from VPC CIDR
  VpcId: !Ref StaticIpVpc
  SecurityGroupIngress:
    - IpProtocol: '-1'
      CidrIp: !Ref VpcCidrBlock
      Description: All Traffic from VPC CIDR

Bonus:
it also helps to add ssh support for debugging:

EC2SecurityGroup:
Type: AWS::EC2::SecurityGroup
Properties:
  VpcId: !Ref StaticIpVpc
  GroupDescription: Access ssh access to
  SecurityGroupIngress:
  - CidrIp : 10.0.0.0/24
    IpProtocol: tcp
    FromPort: 22
    ToPort: 22
  Tags:
  - Key: Name
    Value: EC2SecurityGroup