Your phone probably only has two wlan antennas, meaning 802.11ac/ 2x2 --> 866 MBit/s (theoretical link rate) is the maximum (with any 802.11ac router), with 300-350 MBit/s ending up on your device in practice (the archer c7 is a bit slower than contemporary 802.11ac/ wave2 device, so ~300 MBit/s sounds sensible). For faster connections, your client either needs to support more wifi streams (3x3/ 4x4), which you won't get with common phones/ notebooks - or you'd need to move to 802.11ax routers (which can scratch- and cross the 1 GBit/s barrier in practice).
I think here is a lack of information. To know if you are measuring something reasonable, my suggestion is measure at least WiFi RSSI in both sides, SNR, MCS modulation and channel ocupation in each scenario. And of course be sure that the WiFi is the bottleneck (no the speedtest server, for example).
I think that there is no way to ensure something with "one day somewhere this was 600 Mbps and now here is 300 Mbps"
Honestly, in my experience, 300 Mbps I think is a nice value for a 2x2 80 MHz 802.11ac in a "real case" scenario (as you are not in an anechoic chamber). But this of course still being just an unproofed affirmation because I am talking about "my experience" and not concrete and complete measurements.
In LUCI in "Wireless" you have an easy way to obtain almost all the information I suggested to look:
I gave up. The C7 was causing problems with latest firmware.
Just ordered a AX router, flashed OpenWRT and now get speeds of 500+ MBps with the same laptop and phone.
Not C7 itself, but it's overheating. I'm using it myself in quite some applications and it works just fine IF properly cooled. The cheapest solution is to put a fan towards it But the good laptop blower with HIGH CFM - don't waste your money on a simple and weak ones - also fixes the problem. The ultimate fix is to remove the top cover and attach a heatsink to the SoC, I'm using it with an active cooling. Then it works just fine and perfect: CPU is not throttling itself due to overheat and no problems so far. The network speed is also improving - you can check it. This router is a very good piece of hardware for it's small price, so there's no surprise that some additional work is required to make it work at it's top specs.
But the good laptop blower with HIGH CFM - don't waste your money on a simple and weak ones - also fixes the problem. The ultimate fix is to remove the top cover and attach a heatsink to the SoC, I'm using it with an active cooling. Then it works just fine and perfect: CPU is not throttling itself due to overheat and no problems so far. The network speed is also improving - you can check it. This router is a very good piece of hardware for it's small price, so there's no surprise that some additional work is required to make it work at it's top specs.