Redundant

To be honest, 14 months sounds not bad, but remember that it will be taxed at your highest marginal rate, and ONSS will be deducted.

The law firm Claeys & Engels produced the Claeys formula many years ago and it is typically used in tribunals etc. to establish what a reasonable offer is. You can access it here:
http://www.dismissal.be/ECMS_CLIENT/configuration/pages/home.php

here you can calculate your severance payment
www.preavis.be with all your individual data.

If you are 50+ years old, you are eligible also for a 5,000 EUR outplacement coaching! And I would negotiate another 1-2 months on plus due to your termination during (long?) sick leave (is not kosher).

I would also consult ASAP your mutualité for more info - Belgian law regulates soo much within its labor law, a lawyer (you pay!) is rarely necessary for regular jobs under Belgian regulations.

Paid in cash - if you mean really cash? - do not accept at all - please accept only bank transfers and pay attention that all is paid including social security / tax portions etc.

Good luck, xl

Sorry to hear the news. Reminder: If you want to receive government unemployment, social and healthcare benefits you will need to register with the Belgium unemployment offices in three working days. When you receive your form C4 you must give it to them. Perhaps join a union (ABVV). They can help and advise you.

'I didn't get any improvement plan to turn things around.'
This implies that you have been dismissed and, before being dismissed, you would expect to have been given details of your 'failings' and the chance to improve them.
However, if you have been 'made redundant' that is a totally different situation. Redundancy means that your job/position will no longer exist and so you are no longer needed to fill it. It has nothing to do with your performance but with the fact that your employer no longer needs you - or anyone else - to do the work which you have been doing.
It is typically the result of some internal restructuring of the business.