You should have kept your router, but ...
Although your adapter may be compatible with both G and B, some (if not
all) adapters can be configured to operate in G-only or mixed (they may
also have a B-only mode). Make sure your adapter is set to mixed.
You'll have to use the configuration s/w from the adapter mfr for this.
Ad hoc wireless networks can have a maximum of 9 clients -- so make sure
that you have not exceeded this limit.
The most common problem is that the WEP password isn't entered
correctly. First try connecting without any encryption. If that works,
then enable encryption. You'll probably have to use the hex key rather
than the alphameric password you may have used on other devices. (And,
of course, by going to ad hoc, you've limited encryption to
easily-cracked WEP rather than WPA or WPA2).
If that isn't the problem, you can try setting a static IP address on
the TiVo's adapter. This may or may not work. ICS uses IP addresses in
the 192.168.0.x subnet, and is clever enough to detect static IP
addresses already in use and not try to allocate duplicates. Thus, if
you set the TiVo to something like 192.168.0.15, that ought to be OK.
On the other hand, I seem to recall a TiVo support doc that said that
TiVo does not support using static IP addresses is there is an active
DHCP server on the network. I don't know if the ICS "DHCP allocator" is
enough like a full-fledged DHCP server to cause a problem or not.
Lem -- MS-MVP - Networking
To the moon and back with 2K words of RAM and 36K words of ROM.