My Netgear M2 experience after 1.5 month use - sooooo disappointed

I am sooooooo disappointed with my M2 perfromance recently. My scenario:

- I buy a second hand Austraia Telstra M2 online

- Already updated to latest firmware

- I use without battery

- Usually connecting with normall 2 devices, at most 5

- Using with a 66GB pre-paid sim card

- never use ethernet / USB port

In the first month, the performance is ok, faster than tethering by mobile phone, but in recent two weeks, the performance is sooooo unstable.

- Happened around every 2 days : The router freeze and the blue LED always on, this error is not listed in user manual. I search online but still can't get the answer.

- I tired to turn off the router before sleep and turn on next day morning, but the situation still happen

- Sometime the speed will drop sharply, I tried to turn on the router and check, but the reaction is very slow, I press the ON button but the screen on after few seconds, then I have to reboot.

I use the same SIM during this 1.5 month period, so I am quite sure this is not due to the SIM, and the data is not used up yet. Price of this product I guess is relative high, but the performance is still not stable, I am so disappointed with it.