Connectivity issue with xFi and MacBook Pro

So I recently upgraded my internet from my own personal Netgear modem to an xFi modem as well as increased my bandwidth to the 600 mbps service since I was having issues with my previous service in addition to not having unlimited data which was killing me with two young boys and their iPads.  In any case, the new speeds were a godsend and i could harldy believe my eyes when doing a speed test...  Since installing the new equipment though, I have encountered a few issues, the main one being that my MacBook Pro, which I had no issues with at first, sometimes really lags in the download department.  Now, my MBP is an older model, but that has never had any effect on internet speeds, ever. Now, I understand wifi speeds are slower than being hardwired with an ethernet cable, however, I was easily getting 200-300 mbps downloads speeds over wifi and now I am lucky if I get 10.  I have compared this with my Android device, in the exact same location and get those 200-300 mbps speeds on my phone, but not on my MacBook.  I have rebooted it, checked the network settings, rebooted my router, done speed tests to the gateway(which are over 700mbps every time), and I don't know what else...just has no effect.  I just don't understand what the issue could be.  If I connect it with an ethernet cable, of course it smokes(almost 700mbps), and once I disconnect it, it lags again like with my NetGear modem had been doijng lately(>20mbps).... I have tried the troubleshooting tool through xfinity and it finds no issue with my device.  I have talked to Comcast (Phillipines) and they tell me they can't help me with my device since it's not from Comcast and I can't get a US rep on the phone, it's like trying to talk to God....  I just dont understand why it initially worked so well, and now it acts almost like dial-up...  Now, I will add too that sometimes the xfinity speed test to device gives an error after tapping on Start Test.  I'm not sure that that has anything to do with anything, but it does happen on my Android as well.  Speed test to gateway always seems to work...

Alright, hopefully I described my issue adequately....  Thanks for reading.