Unable to Forward Ports with Bridged Router

Hello, this is an issue that had been plaguing me on Padavan on my Xiaomi Mi 3, and is continuing to do so on my Mi 3G, running PandorasBox.
The bridged router is the TP-Link TL-W9970.

For some odd reason, I simply cannot forward any ports, whether it be for Transmission or ZeroNet,
or something else (basic browsing works as it should).

At first, I had thought that it may be double NAT, since I had run Padavan via IPoE, and leaving the W9970 alone. However the issue persisted, whether I had DHCP running only on the Mi 3, or only on the W9970, and attempting to configure port forwarding on both yielded no fruits.
Funnily enough, port forwarding was working as it should when my laptop was connected directly to the W9970, regardless of where DHCP was running (Padavan automatically set the WAN connection on, so I was able to access the W9970's page even when when it had all it's router settings disabled).

I had put it down to a bug with my copy of Padavan (I had compiled it myself, with Fromeize's script), but no, the same exact issue continues with my Mi 3G on PandorasBox, except that I cannot access the W9970's settings, since I still have not succeeded in setting the same thing via LUCI, without the result just sending me back to the Mi 3G.

I really have no idea what to do now, apart from somehow connecting to the W9970 and making sure (for the 100th time, as I had done before the upgrade) that I had indeed disabled every router feature on the W9970.

Once I get my DM200 working again, I do not believe that I'll face this issue, if the issue at hand is (another) TP-Link firmware bug.

This forum supports OpenWrt, you are running different OSes, from what I understand.

Two links that might help you are:

You are right, however PandorasBox is pretty much OpenWRT with closed-source drivers, and Padavan was temporary.
I would prefer to have OpenWRT proper, it is merely due to driver (and real-life) limitations that I am driven towards PandorasBox for the Mi 3 and Mi 3G, once I unbrick my TP-Link WA850RE and Netgear DM200, they shall run OpenWRT.

Additionally, I am trying to figure out whether this issue is caused by a bug in TP-Link's firmware for the W9970, because if so, I won't be facing this bug on the DM200 once it is up and running.

I have finally repaired my DM200, and flashed OpenWRT on it, so once I figure out how to disable DHCP on my Xiaomi Mi 3G and Mi 3, I'll be able to test this.