Vast difference in WiFi speeds

[homerbronco]

Evening all.

We were switched across to Wifi Max yesterday (white router) from broadband with boost.

When on ISDN we had a rock solid 36mbps on 3 laptops which are all within 10 feet of each other, but since the switch are getting wildly different speeds.
This one is hitting 290+ download (it's the 300 plan) the work laptop bang next to it has achieved 12mbps at best all day (buffering on vid calls, never had before which is what made me do checks) and Mrs laptop on next desk is getting between 18 and 30 download this evening. Two TV's in house, one furthest from Hub (20 foot) loads apps and streams fine, other one running Chromecast 10 foot away won't load apps at all, just spinning circle. Looking at the Sky tests it says all fine (it would and in fairness on one device it is) but drastically different and concerning with the work lappy. Would it be anything to do with the old router was split 2.4 & 5 channels whereas this one is all on one (even if it splits the signal)

Any bright ideas of thoughts on what I can do to change this?

TIA

[Highlinder]

@homerbronco  The Max router you are unable to split the Wi-Fi frequencies.

[homerbronco]

Aye, realise that but wondering if that is the difference as some devices were on 2.4 on old router (others on 5) otherwise can't see a reason for such variances in speeds on 3 similar devices withing 10 feet of each other