[Solved] Cable modem bridging working with DD-WRT, but not with LEDE

Hello everyone.

My previous setup:
LAN <-> TP-Link TL-WR841N v7 on DD-WRT <-> Cable Modem with DHCP <-> WAN

My new setup:
LAN <-> TP-Link Archer C7 v2 on LEDE (17.01.4) <-> Cable Modem with DHCP <-> WAN

That is, I upgraded my hardware and switched from DD-WRT to LEDE.

With the old setup, I can reach the DD-WRT router and connect to forwarded ports. The modem seems to act as a bridge.

With LEDE and the new router, it still looks like the modem does bridging, because I can see in LEDE's overview that the router gets assigned the WAN IP and WAN gateway and there's no double NAT. But port forwarding no longer works and when I portscan the WAN IP from a remote server, the result shows that the device that answers is not my router. Open ports are 135, 139 and 445. It's either the modem itself or some ISP router.

In DD-WRT, there is a setting called "operation mode" and it is set to "Gateway". How can I get LEDE to act in the same way as DD-WRT? I tried restarting router and modem in different order, to no avail. I even tried using the MAC address of the old router.

The cable modem is a Motorola SURFboard SB5101E. I have done a factory reset which didn't help either, so I don't think it's the source of the problem.

Any ideas are appreciated.

Welp, it seems like posting this topic made the problem resolve itself magically, without me doing anything. The wonders of networking.