But it didn't work for BIOS PXE, and neither for UEFI PXE (not sure but almost sure).
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I have seen, with this configuration, TP-LINK is pointing to ipxe.efi, whether I connect from BIOS PXE computer or I connect with UEFI PXE computer. With UEFI profile I see the same error I saw when I tried to connect when I configured only one "config boot 'linux'" section, with 'firmware/ipxe.efi'. With BIOS profile it gives me the same error I saw when I configured only one "config boot 'linux'" section, with 'firmware/ipxe.efi' too, but connecting from BIOS profile. If you need to clarify something, or photos of the screen when I try to boot, please ask. I don't want to overwhelm with too much information.
--EDIT TO HERE--
I don't understand, from the OpenWRT documentation - DHCP , the networkid. In one place it says networkid is to differentiate eth0, eth1, wlan0, etc. But in other place (Booting options) it says: