I have been deleted from all groups, how do I recover them?

I used to have a custom Android ROM running on my xiami redmi 2 phone:
Lineage OS 14.1 Nougat. I can't remember what version of whatsapp I
had, but I suppose it was the most recent when my phone suddenly died
on the 17th of August 2017.

Due to various reasons I've been without smartphone ever since (3
months). Today I purchased a BQ Aquaris M4.5 with Android Marshmallow
running on it. I've installed whatsapp from the play store (version
2.17.395) and have restored my messages from a backup made on the 11th
of August. The very moment I've done so I have been removed from all
the groups in which I was. No private message has been downloaded from
the server either. I thought that it was because, since I am now on
Marshmallow, I might be using a previous version from the one I had
when my old phoned crashed, and have installed the latest version
(2.17.417) but to no avail.

¿Is it there a way to download all the messages sent to me in this
interim, and to make me rejoin all the groups in which I used to be?

Hi, Thanks for your message. The thing is, I restored the backup file that I had on my Google Drive successfully, and all the messages previous to that backup have been retrieved, but I no longer are in all my groups. I can't send messages to the groups and in each group I see the following message: "You can't send messages to this group because you're no longer a participant".

I think you're right that the version of my whatsapp installed is the problem though.

Hi, This is necro posting but I thought I should write what caused this problem so that people in similar situations know.

It was Whatsapp who expelled me from all those groups. It turns out that if your account (given by a phone number) stays offline more than 45 days and then it logs in from another phone, they assume that the phone number of this account has changed of owner, so they delete all data related to the initial account. This is what happened in my case.

Hope this helps.

Pedro

Nope.... in their own words:

After an account has been inactive for 45 days, we have some system security measures in place to protect our user's data. Since it is a common practice for mobile carriers to recycle phone numbers, we monitor account inactivity. If an account is inactive for 45 days and then becomes newly activated on a different mobile device, we take this as a sign that the number has been recycled, and create a brand new account. The original account's information will be deleted. You will lose your groups, account settings, profile photos and status messages,etc.

We have this policy in place to eliminate confusion with recycled phone numbers.

Unfortunately, there is no way to revert this process. If you still wish to be part of the groups you've previously participated in, please ask the group admins to invite you back into the groups.

Hope this helps.