Unable to connect to remote service using AWS VPC Peering

I have a question regarding some AWS infra I’m trying to build. Due to legacy constraints, I have a service running in a remote VPC that I need to access, and I have a new service I’m developing in another VPC. the reason the 2 VPCs are separate is because the newer VPC is managed by Terraform and I am planning to migrate all the services into Terraform but it’s going to take a long time, so for the present time I want to connect the 2 VPCs using AWS VPC Peering (I own both VPCs and both services, I can control anything in this infrastructure, except for the remote service code and deployment details which is a black box).

The remote service I believe is deployed as a single EC2 instance, the new service is deployed using ECS Fargate with an ALB on top. There are 3 security groups involved; the remote instance security group, the ECS security group, and the ALB security group.

I looked at the documentation here as well as a bunch of related pages, and did the appropriate steps. In particular, I did:

• Create the VPC Peering connection
• Accept the peering connection
• Add entries to both VPC routing tables where the destination is the other VPC’s CIDR block and the destination is the ID of the peering connection
• Add a TCP exception to the remote service security group to allow incoming traffic from the ECS security group (by security group ID), and vice-versa
• Check my Network ACL to make sure I have an Allow rule to accept traffic

For some reason, when I try to make my request over the peering connection, the request gets thrown into the void and never returns. It’s not logged on the remote server meaning that I don’t think it ever arrives. I spun up an EC2 instance inside the ECS security group and tried to call the remote resource manually (using curl) and that didn’t work either.

I’ve followed all the guides I could find and can’t figure this out, but I’m a newbie to AWS so maybe there’s something I missed. Does anyone have any other ideas? Thanks.