300Mbs on HH5?

I have set my 2.4MHz wireless to 150 Mbs and my 5GHz to 300 Mbs using the HH5 interface. But my older laptop, which can't use 5GHz and should be connecting at 150 won't see the network. Is it necessary to have 2 different SSIDs under this setup? The 5GHz cards connect very well using 300.

It may not support the wireless security setting if its too long in the tooth. Just a suggestion.

How reliable has the wireless N being on the laptop? Many older devices need updates to drivers to take them from pre-N standard to the proper standard

It was connecting OK to the 2.4 band when both were set to 'up to 150 Mbs'.

A blog post on PC Pro, suggested that the author had to name the two bands with easily identifiable SSIDs. He then only used the 2.4Mhz band as he was getting issues with connectivity on the 5Mhz band; he also ticked a box so that the 2.4 and 5 bands weren't synched.

while I agree having them named differently helps for some devices,
it will make no difference here, - the laptop is 2.4ghz only, and has no concept of the 5ghz network

Rebooted the router? Seems strange!

It should make no difference here. Are you sure you haven't somehow disabled the 2.4ghz network.
By default the 5ghz comes set with speeds above 150mbps I am pretty sure. Why did you change this in the fist instance?

Resetting the router to factory settings should yield the 5ghz speed at max speed and the 2.4 at 150Mbps

The default homehub 5 mode is using a/n/ac at 1300Mbps and doesn't need changing at 5ghz.
Should just work best out of the box without any changing.

It's worth a try separating the SSIDs

I've set two SSIDs and told them not to synch. Now connected at both 2.4 and 5.
I plugged my Netgear dual band dongle into the old laptop and have two connections - the laptop card at 2.4 and the dongle at 5, with two SSIDs. That's either very interesting, or I need to get out more!

So going back a step, does the old laptop work ok with the SSIDs separated?

Yes.

Good - I thought it was a known fix