The chipset of D2 doesn't support WDS on 2.4GHz, If you need to WDS a 2.4GHz network, you bought a wrong product. Confirm with the retailer if you can change it to another device. There are many...

The speed of cable connection or wireless connection to WR741 makes no difference?

If hard rest won't make any difference, it should be faulty. How did you upgrade the router? Is it still under warranty?

It may work but it is not suggested to do so... As TP-LINK has no global warranty policy, if it is faulty in Spain, it would be a headache to get it replaced. Trust me. :)

Link Backup is the right one. If WAN2 is the primary WAN and WAN2 is the backup one, when WAN2 goes down, WAN1 will be activated.