It sounds to me you have a problem with the mechanical gearshift mechanism. Maybe the cable is bad, or stuck, or the internal selector switch has a defect.
what you could check yourself is the following: the gearshift is connected to the gearbox via a cable. Inside the gearbox this cables moves a so-called "selector switch". It can be the case that the cable is too tight (or too loose), causing the selector switch to be in the wrong position.
You can just adjust this under the car, normally, when the gear shift is in "P", there should be a little slack in the outer cables, about 1 mm. If it is more (or less) you can remove a locking from the cable at the gearbox and adjust the tension. See AR4 selector cable.
For a better check you have to remove the oil, take away the carterpan and oil filter(this can be done without removing the gearbox from the car) and put the gearshift in "N". You can see the selectorswitch move now. Change the gears from N to to R, and to D. The movement must now be about 5,5 mm, see AR4 Clearance.
With the last action you need of course help
and you have the advantage that you can check the oil, and renew it along with the filter, that can never hurt. A warning though: there is a special procedure to check the oil level, which can only be done in the Renault garage with an XR25. So measure how much oil you take out and fill with exactly that amount.Expect to have at least 3 liters coming out, if it is less, you should have it checked afterwards at the garage. The AR4 is known to be somewhat critical on the oil level.
Some questions coming up after writing this: can you move the gear shift when the engine is not running? Does it go "heavy"? Do you get a warning light in your dashboard when changing (or trying to change) gears?
Best regards,
Carel