Don't know about Mac but try connecting to the router with a mobile phone using WiFi and check the speed when downloading a webpage or do a speed test with your service provider. Usually most service providers have a speed test link on their webpage so that you are testing directly with their servers and not going through multiple links like when you test using generic online speed tests.
If it is OK the problem is in the Mac.
If it is also slow the problem may be in the internet service as you have changed routers which did not resolve the problem.
Example with a faulty service could be a fault on the line between the service provider and your premises which impacts on speed.
Is there only one connection point for the cable coming into the premises and then the connection to the router or are there multiple cable connection points in the premises and if so is anything connected to them?
What is the internet (and landline phone if applicable) service setup in the premises?
cable in - 1 connection point only - router - computer
same cable in} - connection point - router - computer
same cable in} - multiple connection point - filter - phone
same cable in} - multiple connection point - nothing connected to it.
If you have an ADSL service (landline phone service as well as internet service on same line but phone service is not VOIP i.e. phone not directly connected to modem/router) have you checked the ADSL filter? It should be inserted between the line and the phone to prevent interference in the phone service but can also affect internet service if faulty. If you have this setup, try temporarily disconnecting the filter and the phone from the line and check the speed of the WiFi, both mobile phone and Mac
If you have multiple connection point(s) for the incoming cable that are not being used, disconnect the cable for the unused point(s), where it connects to the connection point that the router is connected to and then test the speed again, both mobile phone and Mac. Hopefully the router is connected to the incoming cable’s 1st connection point and not to a multipled connection point
Hopefully a start.
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