Is it permissible to perform Tayammum and pray Asr sitting down, for fear of missing its time, in the street in a non-Islamic country?

The default for purification for prayer is with water, and it is not permissible to deviate from it to dust unless there is a legitimate Islamic justification. Among the legitimate justifications for performing tayammum (definition) instead of ablution (wudu') is the lack of time, according to some scholars. So, if searching for water or performing ablution would result in the prayer's time expiring, then there is no harm upon you, Allah willing, in performing tayammum. Otherwise, it is not permissible for you to perform tayammum. Your prayer while sitting is not permissible unless you strongly believe that standing would cause you harm, and you cannot find a safe place to pray standing. You must emigrate to a country where you can observe the rituals of your religion if you are unable to do so in the lands of disbelief, and it is not permissible for you to remain in the lands of disbelief without necessity.