Is it permissible to perform ablution and pray, or is it obligatory to forgo prayer and make it up later when one is unable to perform istinja' outside the home?

Impurity must be removed after relieving oneself. The best and most complete way is with water, but it is sufficient to use other things for istinja’ (cleaning oneself), such as tissues or stones, if they remove the impurity, as was the practice of the Prophet (ﷺ) and his companions. There is no excuse for delaying prayer beyond its prescribed time due to an inability to use water for istinja’. Rather, one must remove the impurity with tissues or similar items, then perform ablution and pray.