Port forwarding is doomed for me then as I live in China. The modem/router is in Chinese.
Some modem/routers incorporate a feature known as UPnP.....Universal Plug and Play.....that will automatically open ports as needed. You will likely need to find a tech that can handle Chinese and English to go any further with the modem/router, or any other options, for that matter.
If the modem/router does not have a UPnP feature, or it cannot be set up to forward ports, then your only other option would be to replace the modem/router with a simple modem. A product like this will have only one Ethernet port, like the example below:
If you have a simple modem, then you can set up Port Forwarding on the Time Capsule that connects directly to the simple modem by changing the Router Mode setting of the Time Capsule to DHCP and NAT. The "other" Time Capsule would remain in Bridge Mode.
As far as wireless is concerned, since the Time Capsules are located some distance from each other, you would want to continue to have both of them providing a wireless network signal for better coverage.
Temporarily, power off the "upstairs" Time Capsule work on the "main floor" Time Capsule.
Open AirPort Utility on your Mac
Click on the picture of the main Time Capsule, then click Edit in the smaller window that appears
Click the Wireless tab at the top of the window
Click the Wireless Options button near the bottom of the window
Make sure that a check mark is entered in the box next to 5 GHz Network Name
Click Save, then click Update at the lower right of the next window
Give the Time Capsule a full minute to restart
Then, power up the other Time Capsule and go through the exact same steps
When you do this, the 5 GHz signal will now display the name of the network with "5 GHz" added to the name.
Click the Wifi menu at the top of the Mac's screen
Look for the name of the wireless network that has "5 GHz" added to the name
Select that network to connect your Mac to the 5 GHz network signal
Connect other wireless devices to the 5 GHz network in the same manner.
If you don't want a device to connect to the 5 GHz network, the 2.4 GHz network is still displayed with the normal name of your network (without the 5 GHz added).