I followed exactly this doc
https://openwrt.org/docs/guide-user/network/wifi/relay_configuration
and get the up-link working (I've been going through the doc many times).
But the feature "Wi-Fi extender to be an access point for local devices", after I installed the bridge, does not work here. I cannot get access to this newly created AP - i.e. it looks like it's not getting an IP. Any ideas? Rebooting does also not help..
Just to state the obvious first. relayd is broken by design, it can never work 'well', it should never be used voluntarily - but sometimes it's the only option.
HOWEVER, if both your uplink router and the 'repeater' are running OpenWrt, there is a better options - "WDS/ 4addr", if you can - use that, instead of relayd.
Although it did not solve my problem, thank you for your answer. At least now I know I have to find another solution..
Thank you. I already read before using relayd - it did not help. I aparently might have a special case here, as even standard TP-Link-Repeaters do not work. I have to live with different WLAN IP adresses..
Relayd has always worked fine with me. But to create a true L2 bridge, apart from WDS, (that also works with ipv6) you need Batman-adv which is supported by near everything. It’s a bit more complicated to set up though and requires batman-adv set up on both your main router and aps.
As for relayd, the official guide can be a bit long and confusing. Here’s the easy version:
First set up your SSID/password on your preferred band on your repeater.
Connect with client mode to your main router (which will automatically create the WWAN interface)
Interfaces: make a relaybridge, call it stabridge (or something else). Check to relay between LAN and WWAN.
Firewall LAN: 'Check' WWAN so that both LAN AND WWAN is checked in Covered Networks.
Done. Reboot and your repeater is up.
(Theorethically you would want to make sure internal ip of the repeater is set to another ip/ subnet, in case your router uses the default 192.168.1.0/24, and also turn off dhcp server for the repeaters LAN, but it will work even if you don’t)
I tested again. I only added relayd to my working configuration. Now I can connect to WLAN, but I'm still getting the IP from LAN and the bridge says 'Carrier: absent'.
Should not be necessary as the router to extend uses 192.168.20.0.
Try to disable dhcp server from the Lan on the repeater. I haven’t needed to do that for my limited usage, but it’s good practice to do so. (And something I would anyways do if it’s supposed to be a set it and forget thing).
But tbh it’s not gonna help you if you get carrier absent. Can you try to flash 24.10 and see if it works there? I have also gotten strange carrier absent errors in 25.12, although it’s probably unrelated.
Was already disabled.
I'll test.. Hope I'm not bricking it..
Ok. The only difference in 24.10 is, that the "carrier" is not shown 
I do not understand, how the relayd is supposed to distribute the Subnet of WWAN to my "Master" and why I cannot define IP-Adresses on the "Master".
If even a normal TP-Link repeater fails, I would check the upstream AP/router side too: client isolation, MAC filtering, or a guest network setting can make repeaters look broken. For relayd specifically, keep the repeater LAN on a different management IP than the main router and disable DHCP there. If you can run cable or use WDS/4addr instead, that is still the cleaner fix.