[CentOS] Problem with AT&T VPN

I'm running a CentOS-5.6 server, which in general runs perfectly. My setting is possibly a little unusual: ADSL -> Billion 5200S RC modem/router -> CentOS-5.6 HP MicroServer -> Linksys WRT54GL router .

My daughter is just visiting me, and she has to contact her job using AT&T VPN on her laptop under Windows XP.

Unfortunately this does not work on my system when connected to the Linksys router by WiFi or ethernet. More precisely, the IBM Lotus "sametime" application does not work, and she cannot access the IBM Intranet homepage which she needs to use company tools and applications.

It works perfectly at a public WiFi site nearby.