The fromId argument to get aggTrades refers to the aggregate tradeId which is used no-where else.
The ordinary tradeId is used for fills, 24hr tickers, myTrades, and historicalTrades.
If you have an ordinary tradeId, the only way to get the aggTrades around it requires using the trade’s time to get an aggTrade which has an aggTrade id and then use that with get aggTrade fromId.