TP-Link TL-WR1043ND v1.8 no lan port 4

I dug up two of these old routers recently hoping to use them as a cheap managed switch (no wifi).

However to my surprise, LAN port 4 simply doesn't work on either of them (no link light, no link detected on connected equiptment). One is flashed with 21.02, the other still has what it had last time it was in service (15 iirc).

Restoring stock tp-link firmware makes port4 work again, but afaik doesn't expose enough switch control to do vlan tunking. Even reverting back to Openwrt 10.03 (backfire), port 4 doesn't work!

I'm very surprised I didn't realise this a long time ago!
Any ideas?