1- Making
at-tayammum after relieving oneself - in the event that one is unable to use water - and praying is better than praying with wudu’ while forcefully retaining urine and feces.
2- It is permissible to perform at-tayammum by using a wall, prayer mat etc., if soil or dust is present on them.
3- It is permissible for the person who performs at-tayammum to pray as many obligatory or optional prayers as he wishes, as long as the at-tayammum remains intact.
source:
https://www.al-feqh.com/en