I am not responsible for your wrongdoings or for any damage this may cause. Follow this method at your own risk.
This will erase all the data on your handset. Make sure to backup with Nokia PC Suite before you do this.
Download the hack from the link below.
[symbianzone.us] N97_v22.0.110 Hack RM505_507_22.0.110_uda_4_001.uda.rar
*The hack that you download will have a file name of RM505_507_22.0.110_uda_4_001.uda.fpsx
You may need to rename it according to the same name that your firmware has.
then copy it to the RM-505 folder.
2. Flash your phone using J.A.F. Click here to know how to flash.
3. Restart your phone. Go to applications. Go to Rom Patcher +. Dont' open it yet. Copy the files HERE from below to E:/Patches/
Now your phone has been hacked and you will be able to install full version applications without any certificate error.