Diplomatic immunity is the inability for someone visiting as a diplomat, which would include a US president visiting another country, to be held to a crime or civil penalty, with countries welcome to expel them for abusing this. I don't think that applies.

But a US president who is also a felon could technically be denied correctly by immigration officials, but could reach out to the prime minister to get this fixed, probably in advance.

Diplomatic Immunity is granted by a host country and by the country the diplomat came from. It's not automatically extended. The US historically automatically grants a President diplomat authority but a country can refuse even the highest ranked ambassador if they so choose.

I might be mistaken but whether or not Trump would be admitted to a country with one of these policies it would likely go to a individual vote or decision making authority of whatever governing body runs the country whether or not to grant him a personal exemption due to his political position.

It is also worth mentioning that Trump made some really petty and genuinely awful political decisions that created a lot of hardship for some of the countries on this list. A lot of his wheeling and dealing has been picked apart in courts and actually caused the US some issues since in international trade courts. It may be entirely possible that a country with a grudge would disallow a US president entry which could be quite the setback for the US in multinational bargaining and soft diplomacy.