I must admit, I haven't yet had the need to access the 4G router remotely.
I had heard about the CGNAT issues. For example: ISPReview Forum Thread
(may provide some ideas of a solution).
Also: Another ISPReview Forum Thread
Maybe a Dynamic DNS service (e.g. No-IP) could work? Setting up DDNS on Archer MR200
The thing is, I still use my slow ADSL link (as it does have unlimited data allowance), and combine it with the 4G link, with Load Balancing via the DrayTek. So, for remote access to my LAN, I can use the ADSL link if required. I'd always think of 4G as a "turbo boost" (plus backup link) for the ADSL connection (and if I use all my 4G data before the month is up, I've still got that slower but unlimited data link to fall back on).
It may be possible to find a work-around for the CGNAT (e.g. via a VPN, or other means - another example: CGNAT Workaround
): it's more of an issue if 4G is going to be your *only* internet connection.
Others here may be able to provide further advice.
Good luck with the MR200.