You need to work on your overall rating. If you join one or more of the freelance writing agencies you will be given a rating. The lower the rating the less likelihood you have of receiving well-paid freelance work. The opposite is true. If you can get your rating to the top of the ladder you stand a much better chance of getting well-paid jobs.
What is your reputation in the world of freelance writers? Do you have an excellent reputation? Do the people who have hired you recommend you? You need to get yourself in a position where people are super pleased with the work you've done. The higher your reputation, the greater the demand for your services.
Sometimes it's just a simple matter of charging more. Let's face it if an employer really likes your work and really wants you to work for them and your bid is such and such, there is a good chance that the employer will except that. But what would happen if you said that your fee was more than such and such, even much more? You might find that the employer is quite happy to pay that higher amount because they have such a high opinion of you. You may need to charge more..
Now the big payers are not always the big companies but many of them are. You need to target these companies and advise them of your brilliant services. The bigger companies have a bigger budget and your chances of becoming a well-paid freelance writer may well rest with these groups.
Who are the big players in the freelance writing world? How much do they charge? What sort of services do they provide for those fees? Who pays them? Who employs them? If you know the answer to some or all of these questions there is a much better chance of finding well-paid work.
Networking is surely a great way for you to find better freelance writing jobs. Join a networking group. Let your services be known. If people trust you they will want to employ you.
If you join an agency and it's a huge agency with thousands of freelance writers you may be better off joining more agencies so that there were more possibilities that you get a job.
And getting out there and researching the world of well-paid freelance writing is an excellent idea. Find out who the big players. Find out where the jobs are. Get yourself some better paid freelance writing work.