I am using DHCP on VLAN, but the issue arises when I use Tenda routers, as they display "offered" in the DHCP Leases status.

All other routers are working fine, but the Tenda routers are having issues obtaining an IP address from MikroTik's DHCP server. I have tried connecting using a static IP, but it is not working either, with PPPoE also failing to connect.

However, when I remove the VLAN and set the DHCP server interface to Ether ports, it works fine.

Could someone tell me the possible reason behind this? Is it a MikroTik issue or a Tenda router issue? All Tenda routers are facing the same issue on the VLAN. I have uploaded the network diagram; please check that too and help me out.

Are Tenda routers, on their WAN side, configured to use tagged VLANs? If they are not, then you have to configure access switches (the ones between ONUs and Tendas) and nake Tenda-facing ports as access ports for appropriate VLANs (and keep ONU-facing ports configured as trunk/tagged-only ports).

Yes, the Tenda router is connected to their WAN port.

No there is no VLAN option in the tenda routers, so I set PVID on the TPLINK switch port and then setup DHCP on the tenda router.

I have only tagged the ONU port on the switch with the vlans didn't set any PVID.

Post MT's config. Without it it's not clear what you mean by saying "I am using DHCP on VLAN"...