Trump could recover by 2020. The economy is good, and if he manages to take advantage of that (provided there’s no downturn) it’s possible to imagine him winning re-election. President Obama wasn’t particularly popular in early 2010 and Democrats lost the 2010 midterms in a landslide. But Obama recovered and won re-election by a decent margin in 2012.
Conditions could also get worse for the president. His approval rating on the economy is often higher than his overall approval rating—suggesting that an economic downturn could damage him in the polls. Trump could then struggle on policy, drive numerous persuadable voters toward the Democrats and lose re-election in Carter-esque blowout. Or we could see a scenario somewhere between the middle of the two—e.g. a close loss, another popular vote loss coupled with an electoral college win, etc.
The point here is that we’re a long way away from 2020, and any prediction based on Trump’s approval rating is really just a guess.