Quite long time ago I've configured: Archer C7 v4.0 (fw 1.0.8 Build 20190411 rel.65590(4555)) with RE450 v.3.0 (fw 1.0.1 build 20190124 rel. 56886 (6985)) to have better coverage. Works almost perfectly. Only issue is from time to time need to reset RE450 due to often short power downs in my power grid. Archer is behind UPS which solves the issue.

Recently I bought RE550 ver.1.0 and configured it as Range Extender with same ssid's and passwords as masters (2.4, 5) - same config as RE450.

The network after RE550 it is really slow and the RE450 is no more visible as client on Archer. Looks like some conflict between them.