Access to Admin page very slow to respond.

Using Nighthawk R7960P  Firmware Version V1.4.2.84_1.3.42.

After recent FW update I had many issues (connected to the R7960 via ethernet calbe);

- not able to connect to admin page,

- many IOT devices not able to connect, 

- streaming devices not connecting or buffering

- Smartphones keep cutting out when on home WiFi (iPhone 12, 13, & Galaxy 20, 21)

- Some apps e.g. HBO, Netflix, buffering or not coming up at all. (Amazon Video is the most stable so far)

- Google Nest Hub Max fell off the network and now cannot be connected at all. (google suggested to turn of AP/Client Isolation on Wireless settings. But that setting is not present on my R7960P)

Over last many years i have used various makes and thought NG were the best of all. But now NG is giving me a major headache. 

I have not been able to find solution to any of my issues - at least not the solution that can fix my problem. 

Note: I have two NG extendrs as well (WAX204, Firmware Version:V1.0.2.0 and NightHawk X6S extender)