Thanks for your advice

@RJT463 Thanks for your advice. I made a mistake in the original question. I have P9 system at the moment. Do i have to buy more P9s or can I add to the whole system with M4s. M4s are half the price...

I have the triple pack. It has helped us a lot ironing out our black spots of no Wifi. However we still have a few. If I buy another triple set, will they all work together? Thanks.