I believe that the maximum that VM will allow any customer to have, is three. But, as you already have two and are still experiencing issues, maybe it might be better to stop and re-think the problem?
I assume that the hub is located somewhere towards the front of your house and hence the wifi doesn’t well cover the back, yes? The very best option is to see if you (or an electrician) can run an ethernet cable between the hub and a reasonable location at the back, into which you can connect any third party wireless access point. This would be vastly more reliable than having more boosters.
Remember that they aren’t ‘boosters’ as such, think of it more like a bucket-chain! The hub’s wifi connected to the first pod, which relays the signal on to the next pod and so on. For various technical reasons, each time it does that, it basically halves the speed, unless each pod (or other third party device) can be directly connected to the hub via ethernet cable.
That being said, because of the way they work, each pod needs to be roughly half way* between where the hub physically is and where you have connectivity issues, there’s no point putting one in the problematic room itself. Maybe moving your existing pods around might well help matters? Is there any possibility of relocating the hub itself to a more central location (this would cost you £25 for VM to do, no guarantees, but maybe worth a punt)?
* In theory, true, but complicated by things such as intervening walls, what they are made off, other sources of interference etc.
