Internet signal cutting in/out

Hi there,

Keeping in mind that anyone reading this is very likely to be more educated about routers than I am (so please be patient with me), I have the following issue.

I've had my R8000 router for almost one year now.  Prior to it, I used an Apple Airport router for about 8 years.  Not one day did that router ever produce a single issue or problem!  If it wasn't so out-of-date, I'd have just kept it.  But it is out of date and I'm told, less secure as a result.  Hence the purchase of the R8000 (plus, the R8000 lets me get rid of my Bitdefender Box 2).

I have a T Mobile Home Internet setup where my internet connection is wireless to the nearest 5G tower.  I consistently pull down 150-220 donwload speeds, and 35-75 upload.  Jitter runs in the 5-20 ballpark.  Ping in the 10-30 ballpark.  Packet loss is anywhere from 0-0.5%.  (Not sure if this is useful data or not.

My T Mobile is connected via ethernet cable to the R8000.  And in turn, my two computers are ethernet cable connected to the router.  The router also provides wifi to two iPhones, two iPads, and two smart TVs (with no more than two of those items ever running at the same time).  

Here's my problem I'm hoping I can get help with...   For the longest time (past six months or so), I have issues with my internet connection cutting completely off and on randomely.  The only way to make it stop, is to reboot the router.  But that "fix" only lasts a day or two.  

The T Mobile Modem does have a wifi connection, but I have turned all emitting signals off (and used their forum to be sure that not only are they "off", but that they are not still somehow transmitting).  Unfortunately, that modem has no "bridge mode" option.  And I can't alter the R8000 settings without losing the protection provided by Bitdefender.  In the end, this doesn't seem like it should be a double NAT situation, given that the modem has no ability to transmit wifi now.

Would anyone be able to help me with instructions on what I'm doing wrong?  Or how to keep my internet signal flowing?  I will be honest in saying that I am not a fan of this router, and find it frustratig!  But I don't want to fork out another $200 for another router, when it seems like this one should not be creating problems.  Any assistance would be much appreciated!  Thanks in advance!

Chris