WAN access for two cameras in second subnet

[motxalo]

Hello world,

This is my first post here. I've tried everything I could with my limited knowledge to find the right settings, and this is my last attempt...

I have a rental home setup with two subnets, and two Amcrest IP2M-866W in the second subnet. The second is a smartthings hub sitting behind the WAN router, which in turn is the first subnet providing internet access to guests. All my devices are in the second, away from the main.
I'm attaching a rough diagram.

Question is, how to I need to configure both routers so that I can access the video stream from both cameras remotely?
I have a Synology server at another location setup for accessing video. I've tried a few different configurations without success.

Thanks in advance.
Dani.-