what's OS of your PC? winXP or ??? it can only support Soft AP on win7...

you only installed one adapter and that's why you can't switch.

http://forum.tp-link.com/showthread.php?4310-Failed-to-switch-wireless-network-adapter.&region=china seen by you already. That's the answer why you can't switch because you only installed one...

TL-WA5210G is an access point, so we still need a router which provides internet. Then, try to configure TL-WA5210G working as universal repeater. in that case, you may surf internet by LAN and wifi....

try to configure universal repeater on Tl-WA901ND to extender wireless network from TL-WR741ND.

Try to change another wireless channel on your man router. Or try to reconfigure the RE.