After multiple attempts to identify the root cause of the LLDP issue and the corrupted network topology—problems that appeared after upgrading the Omada Controller to v6—I rebuilt the configuration using different switches and EAPs. In this new setup, everything was functioning correctly. However, once I connected the ER8411 v1 router running firmware v1.3.6 (Build 20251028, Rel.12399), the issues reappeared: the topology became incorrect again and the network state was disrupted .