The spare backup sim on the modem of the maxis router can no longer used on other device. Today it doesn't work on my B618 and also when put in my Oneplus 7. I have put back the back up sim to the ZTE modem on the router. Disconnect my fibre and the backup sim works fine...

So it's not that the sim is banned forever.. its banned to be used other than on the modem+router

Below is the response from maxis CS on twitter

Hi, the 4G SIM Card backup and the dongle can only be used when MaxisONE Home is disconnected or there are service interruptions. While waiting for the Service(s) to be restored, you may be automatically connected to temporary internet through the router wireless backup. You are strictly not allowed to remove the Maxis Equipment (including any SIM Card) from the router without Maxis’written permission. The specific device warranty for the dongle may be void and Service may be barred or terminated if SIM Card is removed from router. TQ-Fiza

Since everyone here is on One Prime. Just get a OneShare line 48 from the principal number and u can get endless data. This way is still legal. I have 4 OneShare line
2 on 4G router
1 my wife main line
1 my father in law put on fon

Alternative. A+B (not yet trying this the same time. A only or B only is working)
A. Use 3rd party router for the home fiber.
B. Use the backup sim inside the provided modem plugged to the maxis router... BRING THIS EVERYWHERE

Then add OneShare48 sim is your best option... Who can give unlimited quota at 48 right?

3. For the oneshare line, can keep any number from maxis or other telco. They will mnp straight become one share line under your name. But if u wanna port out this oneshare line, you must make it principle line 1st for few hours than only do port out.

If one share direct port out confirm fail coz it is a kind of supplementary line

4. Depending on your principle package
Maxis one98 can have 1 oneshare48 line
One188 can have 4 oneshare48 line

Like me
Maxis one prime =
1x One188
4x oneshare48
1x maxis fiber89 which comes with 1backup sim and usb modem

All this in 1 maxis one prime account

This post has been edited by cruxidex: Feb 4 2020, 10:21 PM

Principle and onshare = unlimited quota. In the app it writes as ENDLESS DATA

the 300GB thing is apply to all lines where when the total of all mobile lines (188 and all oneshare) has reached 300GB usage.. the download speeds will become 30mbps only