Route LAN ports to WAN as a switch?

I'm trying to accomplish something that I'm not sure how to approach it and may need some interesting configuration options. Here is my current setup scenario.

I have a TMHI access point that needs to be located in a certain location to provide the best/most stable internet connection. I have an Archer C7 running the latest OpenWRT firmware on it where my Network connections all terminate. I am using one of the wireless radios on the OpenWRT router to access the internet via wifi. I have a couple of hard lined ethernet connections that I have connected to the OpenWRT router that need to be associated with the TMHI network to provide reliable connectivity those devices, as when connected to traditionally to the OpenWRT network, it appears that the access is spotty at best. I'm also using the OpenWRT router for parental control type of features that I also need to keep but not for these ethernet devices.

Is there a way to have the ethernet connections on the OpenWRT network connected to the upstream TMHI router in a switch-like connection, while keeping the existing configuration unchanged? Thanks and let me know if you need any more information.

Any other info I can provide to help with this?

If you want to connect these devices directly on the TMHI router and let OpenWrt connect by wifi, I don't see any issue. But you won't be able to use the OpenWrt as gateway for them anymore.
It is also challenging to connect as wifi client and bridge it with the lan or a wifi access point on OpenWrt, see wds or relay.

The thing is I want to achieve that connection logically, as I cannot physically connect these two devices to the TMHI router. Regarding the wifi, I currently have it running and the OpenWRT client is successfully providing a stable internet connection to the rest of the OpenWRT managed devices. I am trying to find a way to get these two physically connected device to the OpenWRT ethernet LAN ports, to be logically connected to the TMHI router network.

Understood, but it is still hard to do that. With relay the OpenWrt will be a bridge device. With wds it may be possible, but you'll need to bridge the wifi client interface to the lan ports, where these 2 devices are connected, while everything else will still use the wifi client interface as uplink.