I do not think you can achieve what you are trying to do through simple automations or third party solutions like Integromat/Zapier. This is because every time you assign the new enquiry you need to find the lead with the longest date out of all. I think the best solution to achieve this is to build an Airtable script.
The first step would be creating a new column in the advisers table connected to the web enquiries table. This would be the column that links the two tables and makes sure that you can assign a web enquiry to an adviser
create a column that tracks the last time the previously mentioned column is modified
get the script to run in an automation that triggers whenever a new record is added to the web enquiries table. The script should search for the advisor with the longest date from the previous step and match it to the web enquiry
Hope this helps!