How to solve the problem with network brakes when distributing via Time Capsule?

Question

ADSL

I have the last Time Capsule (hereinafter referred to as TC). Distributes the Internet from the ADSL modem. For 1-2 days of modem ping from 1-2 ms becomes 500++ ms.
Reboot TK helps. But this is not the case (the problem comes out at completely unexpected moments).
I also noticed an interesting thing: if the modem's network cable is disconnected from the TC and connected to the laptop for a minute, when it is connected back to the TC, everything becomes ok. Just disconnecting or rebooting the modem does not help!
How to solve the problem without buying a 3rd device (access point)?
PS: I read a trick in Western groups - use a non-WAN port to connect a modem. It does not help!
PPS: TK firmware is the latest.
PPPS: ping to the TC itself is always excellent!