You are welcome anytime!
1.) When you are able to ping your ip over the internet: please ensure that you ping really your router and not the gateway of your mobile provider. In case you are not sure, just let me know.
2.) DynDNS: When you use your router as Gateway, you can use build in DynDNS. When using your Asus as Access-point, I would suggest to use DynDNS of your Modem/Router and forward the needed ports to your Asus. This avoids extra installations of optware-ipkg.
3.) Firmware: Of course you can use Koppel USB-1.71 FW and I can confirm that it works stable. But the FW is now a bit outdated, the newest (and merged) FW is here:
http://code.google.com/p/wl500g/
Here you can choose between "d" branch with kernel 2.4.37 and "rtn" branch with 2.6.22 Kernel. Hence, I suggest to use the newest FW with 2.6 Kernel. The modem part is well implemented and easy to configure. Further, you can use the same optware (ipkg) repository from here:
http://ipkg.nslu2-linux.org/feeds/op.../cross/stable/
4.) And I suggest to install according
Wengis howto
If something is unclear, just ask.
Have fun
Newbiefan